Bump phpunit to .14 for PHP 8.1

Author: barryvdh

composer.json

@@ -28,7 +28,7 @@
         "php-http/mock-client": "^1.1",
         "guzzlehttp/psr7": "^1",
         "mockery/mockery": "^1.3",
-        "phpunit/phpunit": "^8.5|^9"
+        "phpunit/phpunit": "^8.5.14|^9"
     },
     "require-dev": {
         "omnipay/common": "^3",