Bump socket.io from 2.3.0 to 2.4.0

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>

package.json

@@ -32,7 +32,7 @@
     "passport-local": "^1.0.0",
     "passport-local-mongoose": "^6.0.1",
     "qrcode": "^1.4.4",
-    "socket.io": "^3.0.3",
+    "socket.io": "^2.4.0",
     "socket.io-client": "^3.0.3",
     "vue": "~2.6",
     "vue-router": "~3.4.9",