Mac OS 10.13 上 brew update 的权限问题

参考链接：https://github.com/Homebrew/brew/issues/3285
问题：

$ brew update
Warning: /usr/local is not writable.
You should change the ownership and permissions of /usr/local
back to your user account.
  sudo chown -R $(whoami) /usr/local

解决：
re-install homebrew

/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"