fix(package): update firebase-functions to version 3.1.0

Closes #17

Author: greenkeeper[bot]

functions/package.json

@@ -16,7 +16,7 @@
     "cors": "^2.8.4",
     "express": "^4.16.3",
     "firebase-admin": "~6.1.0",
-    "firebase-functions": "^2.0.3",
+    "firebase-functions": "^3.1.0",
     "request": "^2.88.0",
     "request-promise": "^4.2.2"
   },