Use firebase-admin <- @firebase modules

Note: Use dotenv module.

package.json

@@ -8,10 +8,10 @@
   "license": "MIT",
   "private": true,
   "dependencies": {
-    "@firebase/app": "^0.1.2",
-    "@firebase/messaging": "^0.1.3",
     "body-parser": "^1.18.2",
+    "dotenv": "^4.0.0",
     "express": "^4.16.2",
+    "firebase-admin": "^5.5.1",
     "mongoose": "^4.13.2"
   },
   "devDependencies": {