Remove implementations of shouldAutorotateToInterfaceOrientation:

This method was deprecated in iOS 6. As of d4b4d410a5e7c96b230dec1bf8d355b32e4328a9,
the SDK targets iOS 6.0, dropping support for iOS 5.
Merge pull request #75 from box/ios6-deprecation

Remove implementations of shouldAutorotateToInterfaceOrientation: