Fix :head pods

Closes https://github.com/CocoaPods/CocoaPods/issues/2789.
Relies upon https://github.com/CocoaPods/Core/pull/194.

I don't like where that added spec is, open to suggestions.

@neonichu please verify this fixes your issues.

\c @alloy @kylef 
Can you add another dependency that depends on `'AFNetworking/NSURLSession'` to ensure that that does not activate the non-head version or otherwise clash?