Correctly detect new version of xhpast build

Summary: When running lint after xhpast is updated, it tests whether the version is up-to-date. Because the version isn't reset before the test, it would think that the build failed, requiring a second run of lint

Test Plan: Downgraded xhpast, ran lint. Saw broken xhpast, but correct xhpast on second run. Applied patch, and got correct lint results regardless of version.

Reviewers: epriestley, Blessed Reviewers, chad

Reviewed By: epriestley, Blessed Reviewers, chad

Subscribers: Korvin, Firehed

Differential Revision: https://secure.phabricator.com/D15945