Make the output of xhpast --version more compact

Summary: This feels a little nicer, plus is easier to parse.

Test Plan:

> ./scripts/build_xhpast.sh
Build successful!
> ./src/parser/xhpast/bin/xhpast --version
5.5.8/1h

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11639