Ensures HealthKit's status is 'unsupported' on iPad (and other devices that don't support it)

Uses the documented way of checking HealthKit availability.

Note: The linker sets HKHealthKit to nil if the framework is not found (e.g., when running on iOS 7).
https://developer.apple.com/library/ios/documentation/DeveloperTools/Conceptual/cross_development/Using/using.html