Use Carthage for submodules
Fix continuation test not running on main thread
Don't use Carthage for xcconfigs

We always want this to be present, so we don’t want Carthage to
skip it because of its path.
Revert "Don't use Carthage for xcconfigs"

This reverts commit 8806d6b8781b68d696edcdf14b0edae170d5efe6.

Conflicts:
	Carthage.checkout/xcconfigs
Merge pull request #1577 from ReactiveCocoa/cartfile

Use Carthage for submodule management