Merge pull request #61 from reactphp/1.x-update-test-environment-for-PHP-7.2

[1.x] Update test environment for PHP 7.2 to compatible PHPUnit version

Author: clue

composer.json

@@ -34,7 +34,7 @@
         "react/promise": "^3.2 || ^2.1 || ^1.2"
     },
     "require-dev": {
-        "phpunit/phpunit": "^9.6 || ^5.7 || ^4.8.36",
+        "phpunit/phpunit": "^9.6 || ^8.5 || ^5.7 || ^4.8.36",
         "react/async": "^4.3 || ^3 || ^2"
     },
     "autoload": {