Use Zend\Http\Client in Zend\Version

With this update, we can perform version checks without relying on the
environment having allow_url_fopen = 1.

We can now optionally pass in a Zend\Http\Client instance.

Tests were enhanced to run in a separate process to avoid problems during online
tests where tests would not actually be run because the class would cache the
first response, and subsequent responses would use that cached response.

This addresses https://github.com/zendframework/zf2/issues/4610