在 Gemfile 中為 OmniAuth 新增 Gems

gem 'omniauth-oauth2' , '~> 1.3.1'
gem 'omniauth'
gem 'omniauth-facebook'
gem 'omniauth-twitter'
gem 'omniauth-gplus'
gem 'omniauth-linkedin'

執行 bundle install 命令，重新啟動伺服器然後離開！