Bump phpunit/phpunit from 6.5.8 to 6.5.13

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 6.5.8 to 6.5.13.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/6.5.13/ChangeLog-6.5.md)
- [Commits](sebastianbergmann/phpunit@6.5.8...6.5.13)

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

Author: dependabot-support