Update image.ts

Added image._markAsDirty(); on update to cellId
http://www.html5gamedevs.com/topic/34365-gui-imagecellid-doesnt-update-the-image/

gui/src/controls/image.ts

@@ -174,6 +174,7 @@ module BABYLON.GUI {
         }
         set cellId(value: number) {
             this._cellId = value;
+            this._markAsDirty();
         }
 
         constructor(public name?: string, url: Nullable<string> = null) {