Update firebase-admin peer dependency to v5.2.1 (#86)

Author: laurenzlong

package.json

@@ -38,7 +38,7 @@
     "@types/sinon": "^1.16.29",
     "chai": "^3.5.0",
     "chai-as-promised": "^5.2.0",
-    "firebase-admin": "~5.1.0",
+    "firebase-admin": "~5.2.1",
     "istanbul": "^0.4.2",
     "mocha": "^2.4.5",
     "mock-require": "^2.0.1",
@@ -48,7 +48,7 @@
     "typescript": "^2.0.3"
   },
   "peerDependencies": {
-    "firebase-admin": "~5.1.0"
+    "firebase-admin": "~5.2.1"
   },
   "dependencies": {
     "@types/express": "^4.0.33",