attempting to address https://github.com/phinze/puppet-brewcask/issues/6

the brewcask Package provider was failing to find installed apps, leading
it to attempt to install over them. on my system, this was due to the
provider looking at the wrong path. not sure if this is a general solution
or why it was broken, but worked for me.
changed tap from `phinze/homebrew-cask` to `caskroom/homebrew-cask`

not sure why it was pointing to the old repo, but this cleared up some
issues for me, things like:

```
Error: Execution of 'brew tap phinze/homebrew-cask' returned 1: Cloning into '/opt/boxen/homebrew/Library/Taps/phinze/homebrew-cask'...
error: unable to write sha1 filename /opt/boxen/homebrew/Library/Taps/phinze/homebrew-cask/.git/objects/pack/pack-ac9f2c2e4aaabf7eada04554b15cf58503aa6324.pack: No such file or directory
fatal: cannot store pack file
fatal: index-pack failed
```