Update 0.21

This update has several things that has been bugging us lately. Which are:
- https://github.com/RestKit/RestKit/issues/1636 which has been resolved in this commit https://github.com/moneytree/RestKit/commit/0c2e7c82641707a9110838226fa27314c3825152
- Pinning SSL set in the higher level is not getting copied to the individual instance created by the client. https://github.com/moneytree/RestKit/commit/037313982941d416ccd7482d10b5b6f4038873c1

I also reverted commit https://github.com/moneytree/RestKit/commit/492c8a0ab57e6f8ca03bf26c6980c3a73d5dcb70