Raise explicit setup issues about PHP 7

Summary: Ref T9640. Fixes T9888. Decline to support PHP 7 until the async signal handling issue in T11270 is resolved.

Test Plan: Faked local version, got helpful error message.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9640, T9888

Differential Revision: https://secure.phabricator.com/D16231