コマンド gem install rails --no-ri --no-rdoc でレールをインストールしようとすると、エラーが発生します。以下はエラーの出力です
gem install rails --no-ri --no-rdoc
Fetching: multi_json-1.7.8.gem (100%)
Successfully installed multi_json-1.7.8
Fetching: tzinfo-0.3.37.gem (100%)
Successfully installed tzinfo-0.3.37
Fetching: atomic-1.1.12.gem (100%)
Building native extensions. This could take a while...
Successfully installed atomic-1.1.12
Fetching: thread_safe-0.1.2.gem (100%)
Successfully installed thread_safe-0.1.2
Fetching: activesupport-4.0.0.gem (100%)
Successfully installed activesupport-4.0.0
Fetching: builder-3.1.4.gem (100%)
Successfully installed builder-3.1.4
Fetching: rack-1.5.2.gem (100%)
Successfully installed rack-1.5.2
ERROR: While executing gem ... (Gem::RemoteFetcher::UnknownHostError)
no such name (https://rubygems.org/gems/rack-test-0.6.2.gem)
この問題を解決するにはどうすればよいですか?