[CoreFoundation] Hopefully fix a Ruby constant lookup issue.

A few people have reported an issue where the CFAutoRelease constant
could not be found from the singleton method.

* https://github.com/CocoaPods/CocoaPods/issues/2632
* https://github.com/CocoaPods/CocoaPods/issues/2739

I have been unable to reproduce this, but because the issue has been
reported by more than one person, I'm adding this just to be sure.