carthage 0.3 (new formula)

Adds a formula for v0.3 of carthage.

cf https://github.com/Carthage/Carthage/issues/19
`carthage` won’t build with Xcode 6.1 or previous. I’m more or less cargo-culting @Keithbsmiley’s [starting point](https://github.com/Carthage/Carthage/issues/19#issuecomment-63732281).