Could not find nokogiri-1.8.5 in any of the sources

Question

前提・実現したいこと

Ruby on railsでWebサービスを作っています。
UnicornとNginxの設定後、以下のエラーメッセージが発生しました。

発生している問題・エラーメッセージ

 Could not find nokogiri-1.8.5 in any of the sources

該当のソースコード

[ E 2018-12-07 12:57:41.2944 11683/Tf age/Cor/App/Implementation.cpp:221 ]: Could not spawn process for application /home/deploy/merci: The application encountered the following error: Could not find nokogiri-1.8.5 in any of the sources (Bundler::GemNotFound)
  Error ID: 06f0a1f5
  Error details saved to: /tmp/passenger-error-eR42Yy.html

[ E 2018-12-07 12:57:41.2991 11683/T9 age/Cor/Con/CheckoutSession.cpp:276 ]: [Client 1-1] Cannot checkout session because a spawning error occurred. The identifier of the error is 06f0a1f5. Please see earlier logs for details about the error.

試したこと

GemFileに、

gem 'nokogiri', '1.8.5'

を記述し、bundle installを実行