Update firebase-admin dependency to v6 (#213)

Author: laurenzlong

package.json

@@ -40,7 +40,7 @@
     "@types/sinon": "^1.16.29",
     "chai": "^3.5.0",
     "chai-as-promised": "^5.2.0",
-    "firebase-admin": "~5.13.0",
+    "firebase-admin": "~6.0.0",
     "istanbul": "^0.4.2",
     "mocha": "^2.4.5",
     "mock-require": "^2.0.1",
@@ -50,7 +50,7 @@
     "typescript": "~2.8.3"
   },
   "peerDependencies": {
-    "firebase-admin": "~5.13.0"
+    "firebase-admin": "~6.0.0"
   },
   "dependencies": {
     "@types/cors": "^2.8.1",