[LazySpecification] Warn when there are multiple sources per version in #all_specifications
[LazySpecification] Warn when there are multiple sources per version in #all_specifications

Addresses #3 of https://github.com/CocoaPods/CocoaPods/pull/2637#issuecomment-58584402

mglidden/CocoaPods,mglidden/CocoaPods,mglidden/CocoaPods