用命令：sudo gem install -n /usr/local/bin cocoapods
安装cocoapods 提示错误：
ERROR: While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/gems/2.6.0 directory.
截图
提示错误路径未被允许，然后我安装了rbenv ，安装完后再去执行就OK了，为什么出现这种情况，找时间看看！
如何安装rbenv 看这这篇文章https://www.jianshu.com/p/519e41f91b1a
