fix(package): update firebase-admin to version 8.9.1

Closes #12

Author: greenkeeper[bot]

functions/package.json

@@ -15,7 +15,7 @@
     "@js-basics/code-snippet-to-svg": "latest",
     "cors": "^2.8.4",
     "express": "^4.16.3",
-    "firebase-admin": "~6.1.0",
+    "firebase-admin": "~8.9.1",
     "firebase-functions": "^2.0.3",
     "request": "^2.88.0",
     "request-promise": "^4.2.2"