Update phpunit/phpunit requirement to ^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

@@ -41,7 +41,7 @@
     },
     "require-dev": {
         "mikey179/vfsStream": "^1.6",
-        "phpunit/phpunit": "^5.7",
+        "phpunit/phpunit": "^7.0",
         "squizlabs/php_codesniffer": "^3.2"
     }
 }