Make the "PHP 7" setup warning more explicit about what it means

Summary: See https://discourse.phabricator-community.org/t/minimum-php-versions-supported-by-latest-stable-phabricator/2314/3.

Test Plan: o~o

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20027