在cocoapods 执行
sudo gem install cocoapods
的时候出现 While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /usr/bin directory.
改为 sudo gem install -n /usr/local/bin cocoapods 即可
如果是卸载的话，把上面这句改成：
sudo gem uninstall -n /usr/local/bin cocoapods 即可