Carthage integration doesn't work

[lucaslain]

Parse error: expected submodule commit SHA in output of task (ls-tree -z 2.1.0 Example/Carthage/Checkouts/QuizTrain) but encountered: (empty)

[dlewanda]

I was able to come up with a workable solution in #19. I'd appreciate any upvotes to get it merged so we can get back to the mainline project instead of my fork.