fix(package): update firebase to version 7.2.0

Closes #23

Author: greenkeeper[bot]

package.json

@@ -2,7 +2,7 @@
   "name": "react-nextjs-firebase-authentication",
   "version": "0.1.0",
   "dependencies": {
-    "firebase": "^5.5.5",
+    "firebase": "^7.2.0",
     "next": "^7.0.2",
     "next-redux-wrapper": "^2.0.0",
     "react": "^16.6.0",