Update guzzlehttp/guzzle requirement in /build/integration
Updates the requirements on [guzzlehttp/guzzle](https://github.com/guzzle/guzzle) to permit the latest version.
- [Release notes](https://github.com/guzzle/guzzle/releases)
- [Changelog](https://github.com/guzzle/guzzle/blob/6.5.7/CHANGELOG.md)
- [Commits](https://github.com/guzzle/guzzle/compare/6.5.6...6.5.7)