fixes #308: handle dispatch queues properly with ARC on iOS 6+

@@ -130,7 +130,9 @@
         // wait for whole group of fetches (with retries) to finish, then clean up
         //
         dispatch_group_wait(fetchGroup, dispatch_time(DISPATCH_TIME_NOW, NSEC_PER_SEC * self.requestTimeoutSeconds));
+#if ! OS_OBJECT_USE_OBJC
         dispatch_release(fetchGroup);
+#endif
 
         // composite the collected images together
         //

@@ -158,6 +158,10 @@ typedef enum : NSUInteger {
 *   Note that you may want to clear the tile cache after changing this value in order to provide a consistent experience. */
 @property (nonatomic, assign) RMMapboxSourceQuality imageQuality;
 
+#if OS_OBJECT_USE_OBJC
+@property (nonatomic, readonly, strong) dispatch_queue_t dataQueue;
+#else
 @property (nonatomic, readonly, assign) dispatch_queue_t dataQueue;
+#endif
 
 @end

@@ -185,8 +185,10 @@
 
 - (void)dealloc
 {
+#if ! OS_OBJECT_USE_OBJC
     if (_dataQueue)
         dispatch_release(_dataQueue);
+#endif
 }
 
 #pragma mark