当通过 gem install 一些东西的 时候，比如sass ，cocoapods 等经常出现
1
2
ERROR: While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /usr/bin directory.
这样的错误， 都是权限的锅，我们都可以这样处理
1
2
3
sudo gem install -n /usr/local/bin sass
//
sudo gem install -n /usr/local/bin cocoapods
