Add missing dependency for example project (see #3)

Author: zyzo

example/package.json

@@ -44,6 +44,7 @@
     "react": "^0.14.7",
     "react-dnd": "^2.1.2",
     "react-dnd-html5-backend": "^2.1.2",
+    "react-dnd-mouse-backend": "^0.1.0",
     "react-dom": "^0.14.7"
   }
 }