升级mac系统后，brew报错
brew update
Error: Running Homebrew as root is extremely dangerous and no longer supported.
As Homebrew does not drop privileges on installation you would be giving all
build scripts full access to your system.
直接重新安装一个新版本brew即可；
mkdir homebrew && curl -L https://github.com/Homebrew/brew/tarball/master | tar xz –strip 1 -C homebrew
