match presence syntax with V3 database syntax

Author: florianbepunkt

lib/modules/presence.js

@@ -7,7 +7,7 @@ class PresenceRef extends ReferenceBase {
     super(presence.firestack);
 
     this.presence = presence;
-    const db = this.firestack.database;
+    const db = this.firestack.database();
     this.ref = ref;
     this.lastOnlineRef = this.ref.child('lastOnline');
 
@@ -84,7 +84,7 @@ export default class Presence extends Base {
     const path = this.path.concat(key);
     const pathKey = this._presenceKey(path);
     if (!this.instances[pathKey]) {
-      const _ref = this.firestack.database.ref(pathKey);
+      const _ref = this.firestack.database().ref(pathKey);
       this.log.debug('Created new presence object for ', pathKey);
       const inst = new PresenceRef(this, _ref, path);