fix(package): update firebase to version 6.2.1

Closes #15

Author: greenkeeper[bot]

package.json

@@ -3,7 +3,7 @@
   "version": "0.2.0",
   "private": true,
   "dependencies": {
-    "firebase": "^5.6.0",
+    "firebase": "^6.2.1",
     "react": "^16.6.3",
     "react-dom": "^16.6.3",
     "react-router-dom": "^4.3.1",