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