Firebase 6.x.x incompatibility and firebase vs firebase-admin clash.

Feature request: can we please get a geofire version compatible with firebase 6.x.x?

### Version info
UNMET PEER DEPENDENCY firebase@6.6.0
├── firebase-admin@8.5.0
├── firebase-auth@0.1.2
├── firebase-functions@3.2.0
├── firebase-tools@7.3.1
├── geofire@5.0.1
├── object-hash@1.3.1
├── tslint@5.8.0
├── typescript@2.8.4

npm ERR! peer dep missing: firebase@^2.4.0 || 3.x.x || 4.x.x || 5.x.x, required by geofire@5.0.1

### Expected behavior

Compatible geofire library with the latest firebase.

### Actual behavior

The requirement restriction makes it impossible to use "firebase" and 'firebase-admin" together as it shows the known issue where google cloud complains that a "database" has already been registered when these two packages are imported in the same file.

The issue names geofire@5.0.1 and the Firebase packages but no source files or tests. Start by reproducing the dependency and database-registration problems with the listed versions, then inspect the package's peer-dependency declaration and Firebase integration; done means geofire works with Firebase 6.x.x alongside firebase-admin.

Written by the indexing model from the issue text.