Update dependency socket.io to v2 [SECURITY]

Author: renovate[bot]

package.json

@@ -27,7 +27,7 @@
     "passport-twitter": "^1.0.3",
     "sequelize": "^3.5.1",
     "serve-favicon": "^2.3.0",
-    "socket.io": "^1.3.5",
+    "socket.io": "^2.4.0",
     "socket.io-client": "^1.3.5",
     "socketio-jwt": "^4.2.0",
     "sqlite3": "~3.1.1"