This is a little thingy that automatically calls brew update and
optionally brew fetch every half hour and sends you a notification
when something new is available. (It doesn't call upgrade, yet.)
To install, use
brew install petere/auto-update/brew-auto-update --HEAD
and then follow the instructions that are printed:
==> Caveats
To have launchd start petere/auto-update/brew-auto-update now and restart at login:
brew services start petere/auto-update/brew-auto-update
The App Store preferences for checking for updates are respected, so to turn off updating or fetching, change the settings there.