Print a message when lint checks pass, too.

Summary:
That way, we have some visual indication that the linter is actually
running.  We've seen a lot of times it wasn't running for some reason,
and now there's a chance people will notice.

Test Plan: Saw the 'lint ok' message after committing this change.

Reviewers: benkomalo, alpert

Reviewed By: benkomalo

Differential Revision: http://phabricator.khanacademy.org/D3454