Upgrade express version to 4.15.X

Just to make sure this example if free of bugs, such as this one for ms@0.7.1 -> vercel/ms#89

package.json

@@ -10,6 +10,6 @@
     "node": "4.5.x"
   },
   "dependencies": {
-    "express": "4.14.x"
+    "express": "4.15.x"
   }
 }