Bump phpspec/prophecy from 1.8.0 to 1.8.1

Bumps [phpspec/prophecy](https://github.com/phpspec/prophecy) from 1.8.0 to 1.8.1.
- [Release notes](https://github.com/phpspec/prophecy/releases)
- [Changelog](https://github.com/phpspec/prophecy/blob/master/CHANGES.md)
- [Commits](https://github.com/phpspec/prophecy/compare/1.8.0...1.8.1

)

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