[CONTRIBUTING] Add no-nos about littering.
Remove references to Source#data_provider
Merge pull request #2600 from CocoaPods/remove-data-provider

Remove references to Source#data_provider
[TargetIntegrator] Ensures that the user project is not saved when there are no user targets integrated.
Ensure that we raise when attempting to stub an undefined method
[IPC::List] Fix a crash caused by calling an old method
Merge pull request #2601 from CocoaPods/unnecessary-save

[TargetIntegrator] Ensures that the user project is not saved when there are no user targets integrated.
[Installer] Fix issue inheriting pods into empty target

Fixes https://github.com/CocoaPods/CocoaPods/issues/2591
[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'