[MPMD-182] Upgrade to PMD 5.1.1

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1610932 13f79535-47bb-0310-9956-ffa450edef68
[MPMD-188] Require Java 6

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1610933 13f79535-47bb-0310-9956-ffa450edef68