Bump socket.io from 2.2.0 to 2.4.0 in /server

Bumps [socket.io](https://github.com/socketio/socket.io) from 2.2.0 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](socketio/socket.io@2.2.0...2.4.0)

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

Author: dependabot[bot]

server/package-lock.json

@@ -19,7 +19,7 @@
     "node-fetch": "^2.3.0",
     "pg": "^8.7.1",
     "plaid": "^9.3.0",
-    "socket.io": "^2.2.0"
+    "socket.io": "^2.4.0"
   },
   "devDependencies": {
     "eslint": "^6.2.1",