fix phpunit

Author: sokil

composer.json

@@ -14,7 +14,7 @@
         "ext-json": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "^7.5.20",
+        "phpunit/phpunit": ">=7.5.20",
         "php-coveralls/php-coveralls": "^2.1",
         "squizlabs/php_codesniffer": "^3.4",
         "phpmd/phpmd" : "@stable",