Fix checkstyle severities

Summary: "Advice" is not a valid severity for Checkstyle... valid severities are ignore, info, warning and error.

Test Plan: Read the documentation.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D13684