Bump phpunit/phpunit from 6.0.0 to 6.0.8

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 6.0.0 to 6.0.8.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/6.0.8/ChangeLog-6.0.md)
- [Commits](sebastianbergmann/phpunit@6.0.0...6.0.8)

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

Author: dependabot-support