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