carthage 0.4 (new formula)

A work-in-progress for Carthage 0.4 when it is released. Using my fork until my Makefile changes (or something similar) end up upstream.

Currently the test is failing. It shouldn't be so I'll need to look at that.

Follows on from #34677. CC @robrix @Keithbsmiley.
Any ideas on this test failure @robrix? Was seeing hte same locally: http://bot.brew.sh/job/Homebrew%20Pull%20Requests/18104/version=yosemite/testReport/junit/brew-test-bot/yosemite/test_carthage/