Bump phpunit/phpunit from 6.0.6 to 6.5.9

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 6.0.6 to 6.5.9.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-6.5.md)
- [Commits](sebastianbergmann/phpunit@6.0.6...6.5.9)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot-support