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