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