[Dependency] Add a cached prerelease? accessor
[Set] Remove unused resolver-related attributes/method
[Specification] Create subspec dependencies with a version requirement of self.version

@kylef this is one performance optimization for the resolver
[Set] Use #join instead of *
[Specification] Add raise_if_missing parameter to subspec_by_name
[CHANGELOG] Add entry for removal of Specification::Set attributes related to dependency resolution
Merge pull request #186 from CocoaPods/resolver

Resolver