Start leveraging community homebrew cookbook

Made a pass at this in my own scratch space today:
https://github.com/myplanetdigital/myplanet_workstation/commit/dceca7bd400104f226dd39e4389bdf4c35c78466

It's looking like @Govan's critical patch (ie. `node['homebrew']['run_as']`) will hopefully get merged in soon, and from there my patch is just icing -- could easily do the same thing in a `pivotal_workstation` attribute file.

Ref: https://github.com/opscode-cookbooks/homebrew/pull/13

I'll aim to submit a patch for the conversion later, if that works for y'all :)