Lock PHPUnit versions

Author: filips123

composer.json

@@ -31,7 +31,7 @@
         "php": ">= 5.5.9"
     },
     "require-dev": {
-        "phpunit/phpunit": ">= 4.8.36",
+        "phpunit/phpunit": "~4.8 || ~5.7 || ~6.5 || ~7.5",
         "squizlabs/php_codesniffer": "^3.0"
     },
     "autoload": {