Composer: improve PHPUnit version constraints

... as PHPUnit < 5.7.21 doesn't contain the forward compatibility layer with namespaced classes.

Author: jrfnl

composer.json

@@ -18,7 +18,7 @@
         "php": ">=5.3.2"
     },
     "require-dev": {
-        "phpunit/phpunit": "^4.8.36 || ^5.0 || ^6.0 || ^7.0 || ^8.0 || ^9.0",
+        "phpunit/phpunit": "^4.8.36 || ^5.7.21 || ^6.0 || ^7.0 || ^8.0 || ^9.0",
         "php-parallel-lint/php-parallel-lint": "^1.0",
         "php-parallel-lint/php-var-dump-check": "0.*",
         "squizlabs/php_codesniffer": "3.*",