Update maven-checkstyle-plugin, to use Checkstyle 5.6

Removed DoubleCheckedLocking rule from the profile, as this has been
removed in Checkstyle 5.6 release.
See http://checkstyle.sourceforge.net/releasenotes.html for more
information.
Merge pull request #9 from jenkinsci/checkstyle_updates

Update maven-checkstyle-plugin, to use Checkstyle 5.6