Bump phpunit/phpunit from 5.5.4 to 5.6.3 (#11)

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 5.5.4 to 5.6.3.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/main/ChangeLog-12.0.md)
- [Commits](sebastianbergmann/phpunit@5.5.4...5.6.3)

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

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