Bump nikic/php-parser from 5.5.0 to 5.6.0

Bumps [nikic/php-parser](https://github.com/nikic/PHP-Parser) from 5.5.0 to 5.6.0.
- [Release notes](https://github.com/nikic/PHP-Parser/releases)
- [Changelog](https://github.com/nikic/PHP-Parser/blob/master/CHANGELOG.md)
- [Commits](nikic/PHP-Parser@v5.5.0...v5.6.0)

---
updated-dependencies:
- dependency-name: nikic/php-parser
  dependency-version: 5.6.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]