更新ruby时，报错：
Failed to update Homebrew, follow instructions here:
https://github.com/Homebrew/homebrew/wiki/Common-Issues
and make sure `brew update` works before continuing.
Error running 'requirements_osx_brew_update_system ruby-2.4.1',
please read /Users/MACBOOK/.rvm/log/1519807045_ruby-2.4.1/update_system.log
Requirements installation failed with status: 1.
解决如下：
1、更新brew
brew update
ps：如果提示：Error: /usr/local must be writable! 点进去查看解决步骤
2、更新完，再次执行你需要的ruby更新指令
比如：
rvm install 2.4