title: SonarQube doesn't detect rules with new pmd plugin
body: <p>I am using SonarQube v 5.0.1 I have put my rules.xml file in extensions/rules/pmd directory.
  Now everything works if i use  sonar-pmd-plugin version 2.3 - i start sonar, go to rules search and can find all the rules in rules.xml file.
  But after switching to sonar-pmd-plugin version 2.4 i can't find my rules.
  I have switched PMD version in older (v 2.3) plugin and it had no effect so i guess it's not PMD, it's the plugin. Has the default directory of rules changed with latest pmd plugin or something? Couldn't find anything in documentation.</p>