Add an xhpast_version function

Summary: Add a function for retrieving the current XHPAST version number.

Test Plan:

> echo xhpast_version();
xhpast version 5.5.8/1g

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11325