Add the Mailpile beta

See the [release annoucement](https://www.mailpile.is/blog/2014-09-13_Mailpile_Beta_Release.html) for more info.

This replaces caskroom/homebrew-versions#421 since Mailpile [doesn't currently have a stable version](https://github.com/caskroom/homebrew-cask/blob/master/CONTRIBUTING.md#but-there-is-no-stable-version).