Fix memory leak in RMShape by holding a weak reference to the map view.

If there are annotations backed by RMShape on the map view, the map view will not deallocate without this fix.

@@ -44,7 +44,7 @@
 
     NSMutableArray *points;
 
-    RMMapView *mapView;
+    __weak RMMapView *mapView;
 }
 
 @synthesize scaleLineWidth;