安装：
sudo gem install -n /usr/local/bin cocoapods
如果出现：pod :command not found
解决办法
S1：cd /Library/Ruby/Gems/2.0.0/cache/ 里把原来的cocoapods-0.39.0.gem 删掉
S2：sudo chmod 777 /usr/local/bin
S3: sudo gem install -n /usr/local/bin cocoa pods
如果出现：
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/universal-darwin15/rbconfig.rb:213: warning: Insecure world writable dir /usr/local/bin in PATH, mode 040777
解决办法: sudo chmod go-w /usr/local/bin