PHPUnit 6 not compatible

Author: kylekatarnls

composer.json

@@ -28,7 +28,7 @@
     "minimum-stability": "stable",
     "require": {
         "php": ">=5.5.0",
-        "phpunit/phpunit": ">=4.8",
+        "phpunit/phpunit": ">=4.8 <6.0",
         "phpunit/php-code-coverage": ">=2.2",
         "squizlabs/php_codesniffer": "2.*",
         "symfony/console": "^3.2",