Dispatch to main queue, rather than global, fixes #59 (#64)

Need to follow up as to why `ios-test` unit test is failing.  But its not due to reezer's change

platform/ios/platform/ios/src/MGLMapView.mm

@@ -6026,7 +6026,7 @@ public:
 
 - (void)locationManager:(__unused id<MGLLocationManager>)manager didUpdateHeading:(CLHeading *)newHeading
 {
-    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void) {
+    dispatch_async(dispatch_get_main_queue(), ^(void) {
         if ( ! self->_showsUserLocation || self.pan.state == UIGestureRecognizerStateBegan || newHeading.headingAccuracy < 0) return;
 
         self.userLocation.heading = newHeading;