boxen-monkeypatches: fall back to default bottles.

Homebrew provides a bunch of binary packages for things now. When these
have `cellar :any` it means they can be used for installations outside
`/usr/local`.

This commit means that when there isn't a Boxen bottle available it'll
fall back to the Homebrew bottling infrastructure.
Merge pull request #38 from mikemcquaid/use-homebrew-bottles

boxen-monkeypatches: fall back to default bottles.