Use homebrew::tap instead of plain exec

While setupping boxen for my desktop machine, `Exec[tap-homebrew-cask`]
failed for me, because it wasn't able to find `brew` anywhere in the
path.

Most of the times, you should explicitly specify the `path` option,
because people may override the default with `Exec { path => ['...'] }.

I worked around that, not by trying to guess the path in which `brew`
will live, but by using the `homebrew::tap` resource, which takes care
of it all.