Set plugin version and correct ruleset path.
PMD API change 4.3 -> 5.0

Class net.sourceforge.pmd.rules.XPathRule has been renamed to net.sourceforge.pmd.lang.rule.XPathRule

Maven PMD plugin version 3.2 is using PMD 5.1.2
It seems language attribute is mandatory

for a rule - even the XSD doesn't contain it (is it the right XSD?).