[Update] Braintree (2.2.6)

CHANGELOG:                                                                                                
* adds 64-bit support (Thanks @macdrevx, @chrismanderson!)
* bumps iOS Deployment Target from 5.0 to 5.1.1
* use nice constants instead of version numbers. (Thanks @blommegard!)
* internal bug fixes
                                                                                                                
Includes fixes for:
* https://github.com/braintree/braintree_ios/issues/15
* https://github.com/braintree/braintree_ios/pull/18
* https://github.com/braintree/braintree_ios/pull/21
* https://github.com/braintree/braintree_ios/issues/22

Thanks!