Upgrading Database Client (ref-from-ref) (#226)

* Upgrading Database Client (ref-from-ref)

* Updatd integration test

* Upgraded Firestore client

* Bumped database and firebase (test) dependencies

Author: hiranya911

CHANGELOG.md

@@ -1,6 +1,9 @@
 # Unreleased
 
--
+- [changed] Upgraded Realtime Database client to v0.2.0. With this upgrade
+  developers can call the `admin.database().ref()` method with another
+  `Reference` instance as the argument.
+- [changed] Upgraded Cloud Firestire client to v0.13.0.
 
 # v5.9.1