El Capitanにしたあと、

$ brew doctor


Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry and just ignore them. Thanks!

Warning: The /usr/local is not writable.
You should probably `chown` /usr/local

Warning: The /usr/local directory is not writable.
Even if this directory was writable when you installed Homebrew, other
software may change permissions on this directory. Some versions of the
"InstantOn" component of Airfoil are known to do this.

You should probably change the ownership and permissions of /usr/local
back to your user account.
  sudo chown -R $(whoami):admin /usr/local

ディレクトリの所有者の問題でエラーが出てるのでメッセージの通り下記のコマンドを実行して 所有権の変更を行っておきましょう。

$  sudo chown -R $(whoami):admin /usr/local

しかし、、ちがうMacではEl Capitanにアップデートしたらsudoが使えなくなったんだが。。。