Add a version number to ArcanistXHPASTLinter.

Summary: Fixes T5209. It makes sense for the ArcanistXHPASTLinter to implement the getVersion method in order to be able to enforce version requirements (as in T4954).

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5209

Differential Revision: https://secure.phabricator.com/D9317