Update dependency socket.io to ~2.5.0 [SECURITY]

Author: renovate[bot]

package.json

@@ -25,7 +25,7 @@
     "express": "~4.13.3",
     "grunt-cli": "^0.1.13",
     "mustache": "~2.2.1",
-    "socket.io": "~2.0.0",
+    "socket.io": "~2.5.0",
     "underscore": "~1.8.3"
   },
   "devDependencies": {