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)"
