PHPUnit: Update to a modern version

Author: jarstelfox

composer.json

@@ -26,7 +26,7 @@
         "ext-json": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "^4.8.35|^5.7|^6.0",
+        "phpunit/phpunit": "^7",
         "squizlabs/php_codesniffer": "1.*"
     }
 }