chore(package): update firebase to version 7.5.0

Closes #44

Author: greenkeeper[bot]

package.json

@@ -38,7 +38,7 @@
     "@angular/platform-browser-dynamic": "~8.1.1",
     "rxjs": "~6.4.0",
     "zone.js": "~0.9.1",
-    "firebase": "^6.3.0",
+    "firebase": "^7.5.0",
     "@angular-devkit/build-angular": "~0.801.1",
     "@angular/cli": "~8.1.1",
     "@angular/compiler-cli": "~8.1.1",