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