Extend ArcanistXHPASTLinter::LINT_KEYWORD_CASING

Summary: Extend ArcanistXHPASTLinter::LINT_KEYWORD_CASING to include the class keyword.

Test Plan: I can't really added a test case for this without getting "XHP19 Class-Filename Mismatch".

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D10653