Apportable does not yet implement NSManagedObjectContextConcurrencyType.
~/.apportable $ cd ~/.apportable/SDK/System/
~/.apportable/SDK/System $ grep -r NSManagedObjectContextConcurrencyType *
~/.apportable/SDK/System $
If you're not using that part of RestKit, you could comment out the code causing the warning. Otherwise, currently you'd need to reimplement it with other API's.