Drop support for unsupported versions of PHP

[nreynis]

Drop support for PHP < 7.1.
https://www.php.net/supported-versions.php

This will make possible to add strong typing and return types everywhere.

[nreynis]

Also this is probably a requirement to fix #38
I would even go as far as dropping PHP 7.1 who haven´t got much support left anyway so you can upgrade straight to current phpUnit release. Otherwise you´ll be stuck on 7.5.15

[florianeckerstorfer]

Yeah, you're right. Fixed in #40