Fix an issue with PHPCS output formatting

Summary:
See <https://github.com/facebook/arcanist/issues/102>. PHPCS changed its output format sometime between 1.4.6 (stable) and 1.5.0RC3.

Add a "no errors" test and make the linter work on both versions.

Test Plan: Ran `arc unit` on PHPCS 1.5.0RC3.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6926