[Resolver] Take into account versions from all sources (CocoaPods/CocoaPods#2556)
[Resolver] Added tests, notice if the required version exists in multiple sources (CocoaPods/CocoaPods#2556)
[Gemfile] Fetching Core gem version from depending PR https://github.com/CocoaPods/Core/pull/181
[Style] Processed feedback from @segiddins on https://github.com/CocoaPods/CocoaPods/pull/2595
[TargetIntegrator] Ensures that the user project is not saved when there are no user targets integrated.
[Gemfile] Point back to Core master
[Resolver] Use tuned search method to ensure that finding sets is fast
[ResolverSpec] Add a test case for when the second source has the only matching version
Merge branch 'resolver-all-sources'