Use semantic versioning for XHPAST

Summary: Currently XHPAST is versioned as x.y.z/w which seems a little odd. Instead, use x.y.z as the version number to make it possible to use version_compare.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D13960