ERROR: While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /Library/Ruby/Gems/2.0.0 directory.
解决如下：
更新ruby;
$:brew install rbenv ruby-build
$:rbenv global 2.1.2
$:gem update --system,重新打开shell，执行命令
然后：sudo 执行命令
