Bump phpunit/phpunit from 4.8.26 to 4.8.28

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 4.8.26 to 4.8.28.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/4.8.28/ChangeLog-4.8.md)
- [Commits](sebastianbergmann/phpunit@4.8.26...4.8.28)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-type: direct:development
...

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

Author: dependabot[bot]