fix(package): update firebase to version 6.2.2

Closes #114

Author: greenkeeper[bot]

package.json

@@ -8,7 +8,7 @@
     "@material-ui/icons": "^3.0.2",
     "date-fns": "^2.0.0-alpha.27",
     "deepmerge": "^3.2.0",
-    "firebase": "^5.11.0",
+    "firebase": "^6.2.2",
     "i18next": "^15.1.0",
     "prop-types": "^15.7.2",
     "react": "^16.8.6",