chore(package): update firebase to version 5.7.3

Author: greenkeeper[bot]

package.json

@@ -3,7 +3,7 @@
  "version": "0.2.0",
  "private": true,
  "dependencies": {
- "firebase": "^5.6.0",
+ "firebase": "^5.7.3",
  "react": "^16.6.3",
  "react-dom": "^16.6.3",
  "react-router-dom": "^4.3.1",