fix(package): update firebase to version 6.0.3

Closes #12

Author: greenkeeper[bot]

package.json

@@ -4,7 +4,7 @@
   "private": true,
   "dependencies": {
     "date-fns": "^1.30.1",
-    "firebase": "^5.6.0",
+    "firebase": "^6.0.3",
     "react": "^16.6.3",
     "react-dom": "^16.6.3",
     "react-router-dom": "^4.3.1",