Provide an easier way to update the used PMD version

[adangel]

Update the integrated PMD version to xxx.

Ideally, PMD itself would be available in a p2-repository, which the eclipse plugin can then just reference. Given, that the API didn't change, it should be simple then, to update PMD and keep the plugin.

This will however require a bigger change in the plugin and also make sure, that each PMD version is also released into a p2 repository during release.

See: pmd/pmd#1177