Upgrade PHPUnit

composer.json

@@ -41,7 +41,7 @@
         "mikey179/vfsStream": "^1.6",
         "phpmd/phpmd": "^2.6.0",
         "phpspec/prophecy": "^1.8",
-        "phpunit/phpunit": "4 - 5",
+        "phpunit/phpunit": "^6.5",
         "squizlabs/php_codesniffer": "^3.3",
         "symfony/css-selector": "^3.4",
         "symfony/debug": "^3.4",