Merge pull request #43 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

@@ -32,7 +32,7 @@
         "react/promise": "^3 || ^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"
     },
     "autoload": {
         "psr-4": {