更換源
sudo gem update --system
輸入任何 gem 命令都報錯：
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- rubygems/core_ext/kernel_warn (LoadError)?from /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'?from /Library/Ruby/Site/2.3.0/rubygems.rb:1395:in `<top (required)>'?from <internal:gem_prelude>:4:in `require'?from <internal:gem_prelude>:4:in `<internal:gem_prelude>'
解決方法：
gem install bundler
