Allow PHPUnit 8

Author: barryvdh

composer.json

@@ -23,19 +23,20 @@
         "psr-4": { "Omnipay\\Tests\\" : "src/" }
     },
     "require": {
-        "php": "^7.3|^8",
+        "php": "^7.2|^8",
+        "php-http/discovery": "^1.13",
         "php-http/mock-client": "^1.1",
         "guzzlehttp/psr7": "^1",
         "mockery/mockery": "^1.4",
-        "phpunit/phpunit": "^9"
+        "phpunit/phpunit": "^8.5|^9"
     },
     "require-dev": {
         "omnipay/common": "^3",
         "symfony/http-foundation": "^3|^4|^5"
     },
     "extra": {
         "branch-alias": {
-            "dev-master": "4.0.x-dev"
+            "dev-master": "4.1.x-dev"
         }
     },
     "prefer-stable": true