Bump guzzlehttp/guzzle from 6.4.1 to 6.5.2 in /tests/integration

Bumps [guzzlehttp/guzzle](https://github.com/guzzle/guzzle) from 6.4.1 to 6.5.2.
- [Release notes](https://github.com/guzzle/guzzle/releases)
- [Changelog](https://github.com/guzzle/guzzle/blob/master/CHANGELOG.md)
- [Commits](guzzle/guzzle@6.4.1...6.5.2)

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