cocoapods的安装办法，请参见cocoapods官网
sudo gem install cocoapods
如果我们安装失败，并提示ruby 1.9.3 类似的错误，那应该就是最新版的cocoapods不能用ruby 1.9.3来支持，而是ruby 2.0.0。
详细的解决方案见stackoverflow
摘录: Installing new Ruby version helped me. Follow these steps:
rvm reinstall ruby-2.0.0-p247 --with-gcc=clang --verify-downloads 1
gem install xcodeproj
gem install cocoapods
I found this answer in some old Github issue.