Update phpunit/phpunit requirement to ^6.5|^7.0

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-5.7.md)
- [Commits](https://github.com/sebastianbergmann/phpunit/commits/7.0.2)

Signed-off-by: dependabot[bot] <support@dependabot.com>

composer.json

@@ -21,7 +21,7 @@
         "jakub-onderka/php-console-highlighter": "^0.3.2",
         "jakub-onderka/php-parallel-lint": "^1.0",
         "phpunit/php-invoker": "^1.1|^2.0",
-        "phpunit/phpunit": "^6.5",
+        "phpunit/phpunit": "^6.5|^7.0",
         "squizlabs/php_codesniffer": "^3.2"
     },
     "autoload": {