[Resolver] Fix crash when there are lockfile dependencies that were never actually installed (i.e. incompatible platform subspecs)

See https://github.com/CocoaPods/CocoaPods/issues/2789#issuecomment-61797151.
Ensure that HEAD pods are treated properly by using a new Specification::Set subclass
[Rubocop] Fix indentation in installer spec
[InstallerSpec] Add a pod that depends on AFN
Merge pull request #2793 from CocoaPods/seg-fix-head-pods

Fix :head pods