Fix tests on CI

partial revert of 8b3b283612

Easier for travis to fetch from https URL. If you as a developer want to
push to the submodule via git (maintainers only) you'll have to modify
your local git remotes after checkout:

    cd Carthage
    git remote origin set-url git@github.com:WhisperSystems/Signal-Carthage.git

// FREEBIE

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "Carthage"]
 	path = Carthage
-	url = git@github.com:WhisperSystems/Signal-Carthage.git
+	url = https://github.com/WhisperSystems/Signal-Carthage.git