chore(deps): bump socket.io from 2.3.0 to 2.4.0 in /backend

Bumps [socket.io](https://github.com/socketio/socket.io) from 2.3.0 to 2.4.0.
- [Release notes](https://github.com/socketio/socket.io/releases)
- [Changelog](https://github.com/socketio/socket.io/blob/2.4.0/CHANGELOG.md)
- [Commits](https://github.com/socketio/socket.io/compare/2.3.0...2.4.0)

Signed-off-by: dependabot[bot] <support@github.com>

backend/package.json

@@ -30,7 +30,7 @@
     "redis": "^2.8.0",
     "request": "^2.88.0",
     "sha256": "^0.2.0",
-    "socket.io": "^2.2.0",
+    "socket.io": "^2.4.0",
     "underscore": "^1.10.2"
   },
   "devDependencies": {