Handle formula revisions when downloading

This used to manifest in issues like this:

```
brew boxen-install boxen/brews/memcached
==> Installing memcached dependency: libevent
==> Boxen: Downloading http://s3.amazonaws.com/boxen-downloads/homebrew/10.10/libevent-2.0.21.tar.bz2
######################################################################## 100.0%
==> Boxen: Pouring libevent-2.0.21.tar.bz2
Warning: Nothing was installed to /opt/boxen/homebrew/Cellar/libevent/2.0.21_1
Error: /opt/boxen/homebrew/Cellar/libevent/2.0.21_1 is not a directory
```