Bump phpunit/phpunit from 7.5.20 to 8.5.8

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 7.5.20 to 8.5.8.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-8.5.md)
- [Commits](sebastianbergmann/phpunit@7.5.20...8.5.8)

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

Author: dependabot[bot]

composer.json

@@ -16,7 +16,7 @@
         "guzzlehttp/guzzle": "~6.4"
     },
     "require-dev": {
-        "phpunit/phpunit": "~7.5",
+        "phpunit/phpunit": "~8.5",
         "behat/behat": "^3.3"
     }
 }