[maven-release-plugin] prepare release maven-checkstyle-plugin-2.12.1

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1587307 13f79535-47bb-0310-9956-ffa450edef68
[maven-release-plugin] prepare for next development iteration

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1587309 13f79535-47bb-0310-9956-ffa450edef68