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…

…p versions (#13)

Bump dependencies to always support getLastRequest

v3.1.0

Bump mock-client to always support getLastRequest

v3.0-beta.1

Update composer.json