Bump socket.io from 1.7.3 to 2.4.0

Bumps [socket.io](https://github.com/socketio/socket.io) from 1.7.3 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](socketio/socket.io@1.7.3...2.4.0)

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

package.json

@@ -40,7 +40,7 @@
     "redux": "^3.6.0",
     "redux-actions": "^2.0.1",
     "redux-thunk": "^2.2.0",
-    "socket.io": "^1.7.3",
+    "socket.io": "^2.4.0",
     "socket.io-client": "^1.7.3",
     "tether": "^1.4.0"
   },