Update phpunit/phpunit requirement to ~6.5 in /build/integration

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/master/ChangeLog-6.5.md)
- [Commits](https://github.com/sebastianbergmann/phpunit/commits/6.5.8

)

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