升级macOS10.13.4后，CocoaPods遇到的问题（You don't have write permissions...）

升级Xcode10后，使用CocoaPods遇到的一点小问题，做个笔记
终端输入升级指令后，显示

ERROR:  While executing gem ... (Gem::FilePermissionError)

    You don't have write permissions for the /Library/Ruby/Gems/2.3.0 directory.
 

解决办法：

$sudo gem update --system
$gem install -n /usr/local/bin sass