PostgreSQLに接続しないはずなのに、Specified 'postgresql' for database adapterとエラーが出力される

前提・実現したいこと

Railsチュートリアルをローカル環境で進めています。
bundle install --path vendor/bundle --without productionを実行後、
Rails Server起動し、localhost:3000にアクセスするとエラーが出てしまいます。
また、Rails Consoleでも同様のメッセージが出力されました。

エラーメッセージに従い、developmentでもpgのgemを入れればエラーは出力されなくなりましたが、
接続先をSqlite3としているので、なぜPostgreSQLでエラーとなってしまうのかがわかりません。

発生している問題・エラーメッセージ

❯ rails s
=> Booting Puma
=> Rails 5.1.6 application starting in development
=> Run `rails server -h` for more startup options
Puma starting in single mode...
* Version 3.9.1 (ruby 2.4.1-p111), codename: Private Caller
* Min threads: 5, max threads: 5
* Environment: development
* Listening on tcp://0.0.0.0:3000
Use Ctrl-C to stop
Started GET "/" for 127.0.0.1 at 2019-02-03 21:57:06 +0900
Processing by ApplicationController#hello as HTML
Completed 500 Internal Server Error in 235ms



Gem::LoadError (Specified 'postgresql' for database adapter, but the gem is not loaded. Add `gem 'pg'` to your Gemfile (and ensure its version is at the minimum required by ActiveRecord).):

app/controllers/application_controller.rb:5:in `hello'

該当のソースコード

database.yml

# SQLite version 3.x
#   gem install sqlite3
#
#   Ensure the SQLite 3 gem is defined in your Gemfile
#   gem 'sqlite3'
#
default: &default
  adapter: sqlite3
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
  timeout: 5000

development:
  <<: *default
  adapter: sqlite3
  database: db/development.sqlite3

# Warning: The database defined as "test" will be erased and
# re-generated from your development database when you run "rake".
# Do not set this db to the same as development or production.
test:
  <<: *default
  database: db/test.sqlite3

production:
  <<: *default
  database: db/production.sqlite3

source 'https://rubygems.org'

gem 'rails',        '5.1.6'
gem 'puma',         '3.9.1'
gem 'sass-rails',   '5.0.6'
gem 'uglifier',     '3.2.0'
gem 'coffee-rails', '4.2.2'
gem 'jquery-rails', '4.3.1'
gem 'turbolinks',   '5.0.1'
gem 'jbuilder',     '2.6.4'

group :development, :test do
# gem 'pg',      '0.20.0' ←これを書けばエラーで出力されないが、本来不要なはず
  gem 'sqlite3', '1.3.13'
  gem 'byebug',  '9.0.6', platform: :mri
end

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

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

試したこと

rails aboutで確認した情報

❯ rails about
About your application's environment
Rails version        5.1.6
Ruby version         2.4.1-p111 (x86_64-darwin18)
RubyGems version     2.6.11
Rack version         2.0.6
JavaScript Runtime   Node.js (V8)
Middleware           Rack::Sendfile, ActionDispatch::Static, ActionDispatch::Executor, ActiveSupport::Cache::Strategy::LocalCache::Middleware, Rack::Runtime, Rack::MethodOverride, ActionDispatch::RequestId, ActionDispatch::RemoteIp, Sprockets::Rails::QuietAssets, Rails::Rack::Logger, ActionDispatch::ShowExceptions, WebConsole::Middleware, ActionDispatch::DebugExceptions, ActionDispatch::Reloader, ActionDispatch::Callbacks, ActiveRecord::Migration::CheckPending, ActionDispatch::Cookies, ActionDispatch::Session::CookieStore, ActionDispatch::Flash, Rack::Head, Rack::ConditionalGet, Rack::ETag
Application root     /Users/ユーザ名/environment/hello_app
Environment          development

Rubyはrbenvでinstallしたものを使用

❯ which ruby
/Users/ユーザ名/.rbenv/shims/ruby

❯ rbenv local
2.4.1