安装cocoapods（sudo gem install cocoapods）时出现如下错误:
you don't have write permissions for the /system/library/frameworks/ruby.framework/versions/2.6/usr/lib/ruby/gems/2.6.0 directory.
错误截图：
解决方案：
1.安装brew（分别输入如下两行命令，记得每行命令输完按回车）
curl -fsSL -o install.sh https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh
/bin/bash install.sh
2.用brew安装chruby等
brew install chruby ruby-install
3.安装cocoapods
brew install cocoapods
