淘宝镜像：
sudo gem sources -a https://ruby.taobao.org/ sudo gem sources --remove https://rubygems.org/
安装cocoapods
sudo gem update sudo gem install cocoapods -v 0.39 pod setup
install出现错误：
ERROR: Error installing cocoapods:
activesupport requires Ruby version >= 2.2.2.
更新Ruby。
安装rvm
$ curl -L get.rvm.io | bash -s stable
$ source ~/.bashrc
$ source ~/.bash_profile
更Ruby
$ rvm install 1.9.3
