Only build framework schemes

Fixes #124, #143. Tested with the unit test fixture and also the `Cartfile` described in the original issue.

/cc @0xced
I don't think this is Carthage-specific. It certainly aligns with the rest of what's in this file, because we currently _fail_ to build any target that's not a framework—due to our use of settings like `WRAPPER_NAME`, shelling out to `lipo`, etc.