Allow phpunit 6

Author: mateuszsip

composer.json

@@ -24,7 +24,7 @@
         "openlss/lib-array2xml": "~0.0.9"
     },
     "require-dev": {
-        "phpunit/phpunit": "^7.0",
+        "phpunit/phpunit": "^6.0|^7.0",
         "friendsofphp/php-cs-fixer": "^2.4"
     },
     "autoload": {