Update phpunit/phpunit requirement from ~5.6 || ~7.0 to ~5.6 || ~7.0 || ~6.0
Update phpunit/phpunit requirement from ~5.6 || ~7.0 to ~5.6 || ~7.0 || ~6.0

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/6.5.14/ChangeLog-6.5.md)
- [Commits](https://github.com/sebastianbergmann/phpunit/commits/6.5.14)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
LinioIT/tortilla