[Resolver] Fix crash when there are lockfile dependencies that were never…

[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.

lib/cocoapods/resolver.rb

@@ -306,6 +306,9 @@ module Pod
         else
           set = create_set_from_sources(dependency)
         end
+        if dependency.head?
+          set = Specification::Set::Head.new(set.specification)
+        end
         cached_sets[name] = set
         unless set
           raise Molinillo::NoSuchDependencyError.new(dependency) # rubocop:disable Style/RaiseArgs