fix(package): update firebase-admin to version 8.8.0

Closes #26

Author: greenkeeper[bot]

package.json

@@ -36,7 +36,7 @@
   "dependencies": {
     "axios": "^0.18.0",
     "date-fns": "^2.0.0-alpha.27",
-    "firebase-admin": "^7.1.0",
+    "firebase-admin": "^8.8.0",
     "utm": "^1.1.1",
     "firebase-functions": "^2.2.0"
   },