Update dependency firebase-functions to v2.3.1

Author: renovate-bot

package-lock.json

@@ -15,7 +15,7 @@
   },
   "dependencies": {
     "firebase-admin": "7.2.0",
-    "firebase-functions": "2.2.1",
+    "firebase-functions": "2.3.1",
     "next": "8.1.0",
     "react": "16.8.6",
     "react-dom": "16.8.6"