【rails】heroku pushエラー

お世話になっております。
下記のエラーについて知見がある方がいらっしゃいましたらご教示お願いします。

#起きたエラー

$ git push heroku master
Enumerating objects: 28385, done.
Counting objects: 100% (28385/28385), done.
:
（省略）
:
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: -----> Installing bundler 2.0.2
remote: -----> Removing BUNDLED WITH version in the Gemfile.lock
remote: -----> Compiling Ruby/Rails
remote: -----> Using Ruby version: ruby-2.6.6
remote: -----> Installing dependencies using bundler 2.0.2
remote:        Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin -j4 --deployment
remote:        Some gems seem to be missing from your vendor/cache directory.
remote:        Could not find pg-0.20.0 in any of the sources
remote:        Bundler Output: Some gems seem to be missing from your vendor/cache directory.
remote:        Could not find pg-0.20.0 in any of the sources
remote: 
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 secure-oasis-29130.

Herokuにpushした際に上記のエラーが発生しました。

#試したこと1
bundle install --without productionを実行しましたが、エラーは解消されませんでした。

#試したこと２
Installing bundler 2.0.2とエラー文に記載されていたため、
Gemfile.lockとローカルのbundlerのバージョンを2.0.2にしましたがエラーは解消されませんでした。

#関連ファイル

■Gemfile

# frozen_string_literal: true

source 'https://rubygems.org'

gem 'bcrypt', '3.1.12'
gem 'bootstrap-sass', '3.3.7'
gem 'bootstrap-will_paginate', '1.0.0'
gem 'carrierwave', '~> 1.3', '>= 1.3.1'
gem 'coffee-rails', '4.2.2'
gem 'faker', '1.7.3'
gem 'jbuilder',     '2.7.0'
gem 'jquery-rails', '4.3.1'
gem 'jquery-ui-rails'
gem 'pry-rails'
gem 'puma', '3.9.1'
gem 'rails', '5.1.6'
gem 'ransack'
gem 'rubocop', require: false
gem 'rubocop-rails', require: false
gem 'sass-rails', '5.0.6'
gem 'turbolinks', '5.0.1'
gem 'uglifier', '3.2.0'
gem 'will_paginate', '3.1.6'

group :development, :test do
  #  gem 'sqlite3', '1.3.13'
  gem 'byebug', '9.0.6', platform: :mri
  gem 'mysql2'
end

group :development do
  gem 'listen',                '3.1.5'
  gem 'spring',                '2.0.2'
  gem 'spring-watcher-listen', '2.0.1'
  gem 'web-console',           '3.5.1'
end

group :test do
  gem 'guard',                    '2.13.0'
  gem 'guard-minitest',           '2.4.4'
  gem 'minitest',                 '5.10.3'
  gem 'minitest-reporters',       '1.1.14'
  gem 'rails-controller-testing', '1.0.2'
end

group :production do
  gem 'pg', '0.20.0'
end

group :production, :staging do
  gem 'unicorn', '5.4.1'
end

■Gemfile.lock

GEM
  remote: https://rubygems.org/
  specs:
:
(省略)
:
    websocket-extensions (0.1.4)
    will_paginate (3.1.6)

PLATFORMS
  ruby

DEPENDENCIES
  bcrypt (= 3.1.12)
  bootstrap-sass (= 3.3.7)
  bootstrap-will_paginate (= 1.0.0)
  byebug (= 9.0.6)
  carrierwave (~> 1.3, >= 1.3.1)
  coffee-rails (= 4.2.2)
  faker (= 1.7.3)
  guard (= 2.13.0)
  guard-minitest (= 2.4.4)
  jbuilder (= 2.7.0)
  jquery-rails (= 4.3.1)
  jquery-ui-rails
  listen (= 3.1.5)
  minitest (= 5.10.3)
  minitest-reporters (= 1.1.14)
  mysql2
  pg (= 0.20.0)
  pry-rails
  puma (= 3.9.1)
  rails (= 5.1.6)
  rails-controller-testing (= 1.0.2)
  ransack
  rubocop
  rubocop-rails
  sass-rails (= 5.0.6)
  spring (= 2.0.2)
  spring-watcher-listen (= 2.0.1)
  turbolinks (= 5.0.1)
  uglifier (= 3.2.0)
  unicorn (= 5.4.1)
  web-console (= 3.5.1)
  will_paginate (= 3.1.6)

BUNDLED WITH
   2.0.2

■環境
ruby 2.5.1
Rails 5.1.6