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

Closes #131

Author: greenkeeper[bot]

package.json

@@ -13,7 +13,7 @@
     "excel4node": "1.7.0",
     "express": "4.17.1",
     "express-session": "1.16.2",
-    "firebase-admin": "8.4.0",
+    "firebase-admin": "8.9.1",
     "mocha": "6.0.0",
     "moesif-express": "2.9.7",
     "mysql": "2.17.0",