在macOS 10.13上安装cocoapods后pod命令失效，
-bash: /usr/local/bin/pod: /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby: bad interpreter: No such file or directory
解决方法如下：
终端执行
brew link --overwrite cocoapods
尝试 Pod 命令是否已经恢复
若报错继续执行
brew reinstall cocoapods
brew install ruby
brew link --overwrite cocoapods
尝试 Pod 命令是否已经恢复
