carthage 0.3 (new formula)

Adds a formula for v0.3 of carthage.

cf https://github.com/Carthage/Carthage/issues/19
I’m actually not sure that it’s down to the compiler version and not Xcode itself. IIRC the Swift compiler didn’t have any substantive changes, although it _is_ a slightly newer version:

- 6.1:

  ```
  Swift version 1.1 (swift-600.0.54.20)
  Target: x86_64-apple-darwin14.1.0
  ```

- 6.1.1:

  ```
  Swift version 1.1 (swift-600.0.56.1)
  Target: x86_64-apple-darwin14.1.0
  ```

@jspahrsummers, do you recall the nature of the failure to build with Xcode 6.1?