Fix LINT_KEYWORD_CASING for class keyword

Summary: D10653 didn't quite cut it.

Test Plan: Tested on a real file.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D10656