add dependencies for Homebrew formulae

[rolandwalker]

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.

[phinze]

This looks good and I'm for the idea. Does it make more sense to call it depends_on_formula since that more specifically refers to the domain concept inside homebrew? I could see the argument for going hyper-verbose with depends_on_homebrew_formula, but since we're already inside the "homebrew universe" the former seems like enough.

Though if you like brew better I don't feel too strongly. 🌠

[rolandwalker]

I'm glad you like this idea, as I want to move forward with new containers, and #1992 was a dead end. depends_on_formula is obviously better -- I just pushed an amended commit.

However, please don't merge until I add some tests.

[rolandwalker]

I'm giving up on additional tests for now and retreating to my earlier statement that Homebrew provides sufficient coverage here. I end up having to mock Homebrew and then (poorly) duplicating Homebrew's own testing.

I will revisit the testing issues when implementing depends_on_cask, at which time I will perhaps be smarter.

So, this is ready to go in.

[phinze]

Let's do this thing!