Fix ArcanistLesscLinter::getVersion function.

Summary:
This ArcanistLesscLinter::getCacheVersion should have been renamed to getVersion as of D8971, but wasn't updated.

Also fix the regex to properly capture version information.

Test Plan: It's pretty difficult to test this stuff.... I have an idea that I will submit a separate diff for.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9475