carthage 0.3 (new formula)

Adds a formula for v0.3 of carthage.

cf https://github.com/Carthage/Carthage/issues/19
If you remove this and reopen the PR we can/should use a `fails_with` instead which will allows us to list the failing compiler rather than the version of Xcode itself. This makes it easier for people to e.g. fix this up for older OSX versions and send you patches :wink: