尝试用RVM升级Ruby
http://blog.csdn.net/lissdy/article/details/9191351
如果碰到下面问题
Searching for binary rubies, this might take some time.
No binary rubies available for: osx/10.11/x86_64/ruby-2.2.4.
Continuing with compilation. Please read 'rvm help mount' to get more information on binary rubies.
Checking requirements for osx.
Installing requirements for osx.
Updating system.......
Error running 'requirements_osx_brew_update_system ruby-2.2.4',
先运行下面命令，再安装：
rvm autolibs read-only