Check if compiled with iOS 8 SDK

Add another check:

* If you compile with iOS 7 SDK
* Running on iOS 8 or iOS 7

--> Return NO because the implementation is not available

* If you compile with iOS 8 SDK
* Running on iOS 7

--> Return NO because the implementation is not available

* If you compile with iOS 8 SDK
* Running on iOS 8

--> Return if `canOpenUrl:`