Don't break the build when we don't have touch ID stuff

It might be cleaner to just have the old code in -isTouchIDAvailable as before, wrapped in preprocessor directives?

# ifdef __IPHONE_8_0
   LAContext *context = [[LAContext alloc] init];
   return [context canEvaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics error:nil];
# else 
   return NO;
# endif