Update firebase-admin to v5.12.0 (#219)

Author: laurenzlong

package.json

@@ -39,7 +39,7 @@
     "@types/sinon": "^1.16.29",
     "chai": "^3.5.0",
     "chai-as-promised": "^5.2.0",
-    "firebase-admin": "~5.10.0",
+    "firebase-admin": "~5.12.0",
     "istanbul": "^0.4.2",
     "mocha": "^2.4.5",
     "mock-require": "^2.0.1",
@@ -49,7 +49,7 @@
     "typescript": "^2.0.3"
   },
   "peerDependencies": {
-    "firebase-admin": "~5.11.0"
+    "firebase-admin": "~5.12.0"
   },
   "dependencies": {
     "@types/cors": "^2.8.1",