Bump minimum PHPUnit and CS Fixer required version

Author: Jean85

composer.json

@@ -31,10 +31,10 @@
         "symfony/yaml": "^3.0||^4.0"
     },
     "require-dev": {
-        "friendsofphp/php-cs-fixer": "^2.5",
+        "friendsofphp/php-cs-fixer": "^2.8",
         "phpstan/phpstan": "^0.9.1",
         "phpstan/phpstan-phpunit": "^0.9.1",
-        "phpunit/phpunit": "^6.0",
+        "phpunit/phpunit": "^6.5",
         "scrutinizer/ocular": "^1.4",
         "symfony/expression-language": "^3.0||^4.0"
     },