Improve the handling of the php_compat_info.json file.

Summary: See inline comments on D9576. Also replace json_decode with phutil_json_decode, for better error handling.

Test Plan: Make sure arc unit still works.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9656