[MPMD-176] use pmd 5.1.0

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1568052 13f79535-47bb-0310-9956-ffa450edef68
[MDEPLOY-174] [REGRESSION] url not required anymore, causing a NullPointerException
Fix required-marker and @since doclettags which were lost after MDEPLOY-151

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1568656 13f79535-47bb-0310-9956-ffa450edef68