composer.json php-code-coverage update to ^6.0

Author: ek9

composer.json

@@ -26,7 +26,7 @@
     },
     "require": {
         "php": ">=5.6",
-        "phpunit/php-code-coverage": "^5.0,<6.0",
+        "phpunit/php-code-coverage": "^6.0",
         "behat/behat": "^3.0",
         "guzzlehttp/guzzle": "^6.0",
         "symfony/config": "^2.3||^3.0||^4.0",