Herokuでrailsアプリのデプロイでエラー

rails初心者です。

HEROKUでrailsアプリのデプロイを試みているのですがエラーが出てしまいデプロイができません。
色々調べて試したのですが原因がわかりません。
出ているエラーは下記になります。

git push heroku master

Counting objects: 194, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (172/172), done.
Writing objects: 100% (194/194), 52.05 KiB | 2.74 MiB/s, done.
Total 194 (delta 31), reused 0 (delta 0)
remote: Compressing source files... done.
remote: Building source:
remote: 
remote:  !     Warning: Multiple default buildpacks reported the ability to handle this app. The first buildpack in the list below will be used.
remote:             Detected buildpacks: Ruby,Node.js
remote:             See https://devcenter.heroku.com/articles/buildpacks#buildpack-detect-order
remote: -----> Ruby app detected
remote: -----> Compiling Ruby/Rails
remote: -----> Using Ruby version: ruby-2.3.4
remote: -----> Installing dependencies using bundler 1.15.2
remote:        Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin -j4 --deployment
remote:        Your Gemfile lists the gem mysql2 (= 0.3.18) more than once.
remote:        You should probably keep only one of them.
remote:        While it's not a problem now, it could cause errors if you change the version of one of them later.
remote:        Warning: the running version of Bundler (1.15.2) is older than the version that created the lockfile (1.16.0). We suggest you upgrade to the latest version of Bundler by running `gem install bundler`.
remote:        Fetching gem metadata from https://rubygems.org/..........
remote:        Fetching version metadata from https://rubygems.org/..
remote:        Fetching dependency metadata from https://rubygems.org/.
remote:        The latest bundler is 1.16.0, but you are currently running 1.15.2.
remote:        To update, run `gem install bundler`
remote:        Could not find rack-test-0.7.1 in any of the sources
remote:        Bundler Output: Your Gemfile lists the gem mysql2 (= 0.3.18) more than once.
remote:        You should probably keep only one of them.
remote:        While it's not a problem now, it could cause errors if you change the version of one of them later.
remote:        Warning: the running version of Bundler (1.15.2) is older than the version that created the lockfile (1.16.0). We suggest you upgrade to the latest version of Bundler by running `gem install bundler`.
remote:        Fetching gem metadata from https://rubygems.org/..........
remote:        Fetching version metadata from https://rubygems.org/..
remote:        Fetching dependency metadata from https://rubygems.org/.
remote:        The latest bundler is 1.16.0, but you are currently running 1.15.2.
remote:        To update, run `gem install bundler`
remote:        Could not find rack-test-0.7.1 in any of the sources
remote:  !
remote:  !     Failed to install gems via Bundler.
remote:  !
remote:  !     Push rejected, failed to compile Ruby app.
remote: 
remote:  !     Push failed
remote: Verifying deploy...
remote: 
remote: !    Push rejected to cose-app.
remote: 
To https://git.heroku.com/cose-app.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/cose-app.git'

#gemfile

source 'https://rubygems.org'

git_source(:github) do |repo_name|
  repo_name = "#{repo_name}/#{repo_name}" unless repo_name.include?("/")
  "https://github.com/#{repo_name}.git"
end


# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '~> 5.1.4'
# Use mysql as the database for Active Record
gem 'mysql2', '0.3.18'
# Use Puma as the app server
gem 'puma', '~> 3.7'
# Use SCSS for stylesheets
gem 'sass-rails', '~> 5.0'
# Use Uglifier as compressor for JavaScript assets
gem 'uglifier', '>= 1.3.0'
# See https://github.com/rails/execjs#readme for more supported runtimes
# gem 'therubyracer', platforms: :ruby

# Use CoffeeScript for .coffee assets and views
gem 'coffee-rails', '~> 4.2'
# Turbolinks makes navigating your web application faster. Read more: https://github.com/turbolinks/turbolinks
gem 'turbolinks', '~> 5'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.5'
# Use Redis adapter to run Action Cable in production
# gem 'redis', '~> 3.0'
# Use ActiveModel has_secure_password
# gem 'bcrypt', '~> 3.1.7'

# Use Capistrano for deployment
# gem 'capistrano-rails', group: :development

group :development, :test do
  # Call 'byebug' anywhere in the code to stop execution and get a debugger console
  gem 'byebug', platforms: [:mri, :mingw, :x64_mingw]
  # Adds support for Capybara system testing and selenium driver
  gem 'capybara', '~> 2.13'
  gem 'selenium-webdriver'
end

group :development do
  # Access an IRB console on exception pages or by using <%= console %> anywhere in the code.
  gem 'web-console', '>= 3.3.0'
  gem 'listen', '>= 3.0.5', '< 3.2'
  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring'
  gem 'spring-watcher-listen', '~> 2.0.0'
end

group :production do
  gem 'rails_12factor'
  gem 'mysql2', '0.3.18'
end

# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]
gem 'pry-rails'
gem 'devise'
gem 'enumerize'
gem 'bootstrap-sass'
gem 'jquery-rails'
gem 'cancancan'
gem 'rails_admin'
gem "chartkick"
gem 'ransack'
gem 'momentjs-rails'
gem 'bootstrap3-datetimepicker-rails'
gem 'kaminari'

お詳しい方ご教授お願い致します。

以下追記

config/environments/production.rb

Rails.application.configure do
  # Settings specified here will take precedence over those in config/application.rb.

  # Code is not reloaded between requests.
  config.cache_classes = true

  # Eager load code on boot. This eager loads most of Rails and
  # your application in memory, allowing both threaded web servers
  # and those relying on copy on write to perform better.
  # Rake tasks automatically ignore this option for performance.
  config.eager_load = true

  # Full error reports are disabled and caching is turned on.
  config.consider_all_requests_local       = false
  config.action_controller.perform_caching = true

  # Attempt to read encrypted secrets from `config/secrets.yml.enc`.
  # Requires an encryption key in `ENV["RAILS_MASTER_KEY"]` or
  # `config/secrets.yml.key`.
  config.read_encrypted_secrets = true

  # Disable serving static files from the `/public` folder by default since
  # Apache or NGINX already handles this.
  config.public_file_server.enabled = ENV['RAILS_SERVE_STATIC_FILES'].present?

  # Compress JavaScripts and CSS.
  config.assets.js_compressor = :uglifier
  # config.assets.css_compressor = :sass

  # Do not fallback to assets pipeline if a precompiled asset is missed.
  config.assets.compile = true

  # `config.assets.precompile` and `config.assets.version` have moved to config/initializers/assets.rb

  # Enable serving of images, stylesheets, and JavaScripts from an asset server.
  # config.action_controller.asset_host = 'http://assets.example.com'

  # Specifies the header that your server uses for sending files.
  # config.action_dispatch.x_sendfile_header = 'X-Sendfile' # for Apache
  # config.action_dispatch.x_sendfile_header = 'X-Accel-Redirect' # for NGINX

  # Mount Action Cable outside main process or domain
  # config.action_cable.mount_path = nil
  # config.action_cable.url = 'wss://example.com/cable'
  # config.action_cable.allowed_request_origins = [ 'http://example.com', /http:\/\/example.*/ ]

  # Force all access to the app over SSL, use Strict-Transport-Security, and use secure cookies.
  # config.force_ssl = true

  # Use the lowest log level to ensure availability of diagnostic information
  # when problems arise.
  config.log_level = :debug

  # Prepend all log lines with the following tags.
  config.log_tags = [ :request_id ]

  # Use a different cache store in production.
  # config.cache_store = :mem_cache_store

  # Use a real queuing backend for Active Job (and separate queues per environment)
  # config.active_job.queue_adapter     = :resque
  # config.active_job.queue_name_prefix = "cose_#{Rails.env}"
  config.action_mailer.perform_caching = false

  # Ignore bad email addresses and do not raise email delivery errors.
  # Set this to true and configure the email server for immediate delivery to raise delivery errors.
  # config.action_mailer.raise_delivery_errors = false

  # Enable locale fallbacks for I18n (makes lookups for any locale fall back to
  # the I18n.default_locale when a translation cannot be found).
  config.i18n.fallbacks = true

  # Send deprecation notices to registered listeners.
  config.active_support.deprecation = :notify

  # Use default logging formatter so that PID and timestamp are not suppressed.
  config.log_formatter = ::Logger::Formatter.new

  # Use a different logger for distributed setups.
  # require 'syslog/logger'
  # config.logger = ActiveSupport::TaggedLogging.new(Syslog::Logger.new 'app-name')

  if ENV["RAILS_LOG_TO_STDOUT"].present?
    logger           = ActiveSupport::Logger.new(STDOUT)
    logger.formatter = config.log_formatter
    config.logger    = ActiveSupport::TaggedLogging.new(logger)
  end

  # Do not dump schema after migrations.
  config.active_record.dump_schema_after_migration = false
end