CB-11838 ios: Unregister callback function at the right timing.

We at LINE observed about 40K crashes a day that were suspected to be
caused by the reacahability callback function invoked on an
already-deallocated object. We couldn't reproduced the crash locally
but this patch did reduce the number of crash reports to zero.

 This closes #49

src/ios/CDVReachability.m

@@ -120,6 +120,7 @@ static void CDVReachabilityCallback(SCNetworkReachabilityRef target, SCNetworkRe
 - (void)stopNotifier
 {
     if (reachabilityRef != NULL) {
+        SCNetworkReachabilitySetCallback(reachabilityRef, NULL, NULL);
         SCNetworkReachabilityUnscheduleFromRunLoop(reachabilityRef, CFRunLoopGetCurrent(), kCFRunLoopDefaultMode);
     }
 }