add dependencies for Homebrew formulae

uses new DSL key `depends_on_brew`.

Pursuant to #1992 and relevant to discussion in #2264, it was the request of @phinze on IRC *not* to add `depends_on` stanzas to `brew-cask.rb`, but to defer dependencies until install-time for individual Casks.

This means adding to the DSL so we can specify which Casks have dependencies.  I have chosen the key `depends_on_brew` to leave room for specifying dependencies between Casks at a later time.  Homebrew will handle the dependency chain if the given dependency has dependencies of its own within Homebrew.

I'm not familiar with the test suite at this point. I wouldn't mind some help/pointers on tests for some of my PRs.  For this particular case, homebrew's own test suite already has good coverage of the principal functionality.