[maven-release-plugin] prepare release maven-invoker-plugin-1.8.1

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1606209 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@1606211 13f79535-47bb-0310-9956-ffa450edef68