Change PHP 7 setup warning to complain about 7.0 only, not 7.1+

Summary: Ref T9640. On 7.0 we had signal handling issues so we can never support it, but async signals should resolve them on 7.1 or newer.

Test Plan: On PHP 7.1, got through the setup warning.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9640

Differential Revision: https://secure.phabricator.com/D17197