Update dependency socket.io to v2 [SECURITY]

package.json

@@ -23,6 +23,6 @@
     "promise": "^7.0.1",
     "semver": "^4.3.3",
     "serve-favicon": "~2.1.3",
-    "socket.io": "^1.3.5"
+    "socket.io": "^2.4.0"
   }
 }