Update firebase-admin in integration tests to v5.12.0 (#229)

Author: swalkowski

integration_test/functions/package.json

@@ -8,7 +8,7 @@
     "@google-cloud/pubsub": "^0.6.0",
     "@types/lodash": "^4.14.41",
     "firebase": "^4.9.1",
-    "firebase-admin": "~5.10.0",
+    "firebase-admin": "~5.12.0",
     "firebase-functions": "./firebase-functions.tgz",
     "lodash": "^4.17.2"
   },