fix(package): update firebase-admin to version 8.9.0

Closes #26

Author: greenkeeper[bot]

functions/package.json

@@ -17,7 +17,7 @@
   "dependencies": {
     "axios": "^0.18.0",
     "date-fns": "^2.0.0-alpha.27",
-    "firebase-admin": "~7.1.0",
+    "firebase-admin": "~8.9.0",
     "firebase-functions": "^2.1.0",
     "utm": "^1.1.1"
   },