[Set] Sources are used in the order in which they are provided
[Style] Fixes two RuboCop offenses
[Specification::Source::Aggregate] Updates spec for https://github.com/CocoaPods/Core/pull/181
[Set] Fix for relying on hash key order as the order of sources
[Source::Aggregate] Fix for relying on hash key order for the sources provided to Sets
[Style] Processed feedback from @segiddins on https://github.com/CocoaPods/Core/pull/181
Merge pull request #181 from Thomvis/issue/CocoaPods/2556

[Set] Sources are used in the order in which they are provided