Staging release 6.8.0 (firebase#260)

* Updating CHANGELOG for 6.8.0 release.

* [maven-release-plugin] prepare release v6.8.0

* [maven-release-plugin] prepare for next development iteration

Author: hiranya911

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Unreleased
 
+-
+
+# v6.8.0
+
 - [added] Implemented new `sendAll()` and `sendMulticast()` APIs in
   `FirebaseMessaging`.
 - [changed] Removed org.json dependency and replaced with com.google.code.gson.

pom.xml

@@ -19,7 +19,7 @@
 
     <groupId>com.google.firebase</groupId>
     <artifactId>firebase-admin</artifactId>
-    <version>6.7.1-SNAPSHOT</version>
+    <version>6.8.1-SNAPSHOT</version>
     <packaging>jar</packaging>
 
     <name>firebase-admin</name>