更换源
sudo gem update --system
输入任何 gem 命令都报错：
/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>'
解决方法：
gem install bundler
