[maven-release-plugin] prepare release jackrabbit-oak-0.15

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1560292 13f79535-47bb-0310-9956-ffa450edef68
[maven-release-plugin] prepare for next development iteration

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1560294 13f79535-47bb-0310-9956-ffa450edef68