Bump phpunit to version that supports PHP8.1

Bump to PHPUnit 8.5.14 to support 8.1 in the lowest matrix; https://github.com/sebastianbergmann/phpunit/blob/main/ChangeLog-8.5.md#8514---2021-01-17

v4.1.1

Enable Mock strategy for HttpRequests

Allow phpunit 7 and PHP 7.2 again

v4.1.0

Update composer.json

v4.0.2

Update GatewayTestCase.php

Fix IsInternalType

v4.0.1

uniqid() more entropy

v4.0.0

Update composer.json

Allow Guzzle Adapter 2.x

v3.1.1

Allow guzzle6 v2

v2.1.1

permit higher phpunit to be used. increases compatibility with new ph…

Bump dependencies to always support getLastRequest

v3.1.0

Bump mock-client to always support getLastRequest

v3.0-beta.1

Update composer.json