Update firebase requirement from ^3.4.0 to ^6.3.0

Updates the requirements on [firebase](https://github.com/firebase/firebase-js-sdk) to permit the latest version.
- [Release notes](https://github.com/firebase/firebase-js-sdk/releases)
- [Commits](https://github.com/firebase/firebase-js-sdk/compare/rxfire@3.4.0...firebase@6.3.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

package.json

@@ -7,7 +7,7 @@
   },
   "dependencies": {
     "eslint-config-rallycoding": "^3.1.0",
-    "firebase": "^3.4.0",
+    "firebase": "^6.3.0",
     "react": "15.3.2",
     "react-native": "0.34.0"
   }