Fix ruleset path

When running phpcs on a projects which was using the 'master' ruleset before the split into four repositories, I ran into the following error:
`Fatal error: Uncaught exception 'PHP_CodeSniffer_Exception' with message 'Referenced sniff "CodeSniffer/Standards/WordPress/Subsets/imports.ruleset.xml" does not exist' in path/to/PEAR/PHP/CodeSniffer.php:847`

This PR fixes that.