wordpress needs phpunit 6

composer.json

@@ -12,7 +12,7 @@
         "monolog/monolog": "^1.23"
     },
     "require-dev": {
-        "phpunit/phpunit": "^7.4",
+        "phpunit/phpunit": "^6",
         "wp-coding-standards/wpcs": "^0.14.1",
         "squizlabs/php_codesniffer": "^3.0.2",
         "10up/wp_mock": "^0.3",