Allow PHP version to be customized with ArcanistXHPASTLinter

Summary: Fixes T5385. Provide a flexible means of setting a minimum PHP version for the ArcanistXHPASTLinter, instead of relying on ArcanistXHPASTLinter::LINT_PHP_53_FEATURES and ArcanistXHPASTLinter::LINT_PHP_54_FEATURES.

Test Plan: Fixed up and ran unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5385

Differential Revision: https://secure.phabricator.com/D9576