Decrease the priority of the ArcanistPhutilLibraryLinter

Summary: Ref T5577. Decrease the priority of the ArcanistPhutilLibraryLinter such that it is lower than that of the ArcanistXHPASTLinter. There isn't any specific reason that we should do this, although it seems to generally be a reasonable idea.

Test Plan: N/A

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5577

Differential Revision: https://secure.phabricator.com/D9866