我试了下可以下载其他的gem，但是在下载cocoapods的时候报
undefined method `size' for nil:NilClass (NoMethodError) in /Library/Ruby/Gems/2.0.0/cache/cocoapods-0.39.0.gem
说是我原来的 /Library/Ruby/Gems/2.0.0/cache/ 里已经存在了cocoapods-0.39.0.gem，导致在解压的时候失败了
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
feel happy~