Only try to ask user permission if device is running iOS 8 or above. Fix crash.

Only try to ask user permission is device is running iOS 8 or above. Fix crash when asking a user for notification permission on versions below iOS 8.

Previously, if this method was called on a non iOS 8 device, the app crashed. Please see: https://github.com/katzer/cordova-plugin-local-notifications/issues/261

This way the permission is only called on devices that are running iOS 8 or greater.