phpunit 6.5 is the last one to work with hhvm

Author: iamcal

composer.json

@@ -15,7 +15,7 @@
 		}
 	},
 	"require-dev": {
-		"phpunit/phpunit": ">=4.8",
+		"phpunit/phpunit": "~6.5",
 		"php-coveralls/php-coveralls": "~1.0"
 	}
 }