Use XHPAST version instead of checksum as cache version

Summary: Rely on the output of xhpast --version when determing the lint cache key.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11611