Minor change to update_compat_info.php

Summary: Replace file_put_contents with Filesystem::writeFile.

Test Plan: Ran ./scripts/update_compat_info.php.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9960