Bump phpspec/prophecy-phpunit from 2.3.0 to 2.4.0

Bumps [phpspec/prophecy-phpunit](https://github.com/phpspec/prophecy-phpunit) from 2.3.0 to 2.4.0.
- [Release notes](https://github.com/phpspec/prophecy-phpunit/releases)
- [Changelog](https://github.com/phpspec/prophecy-phpunit/blob/master/CHANGES.md)
- [Commits](phpspec/prophecy-phpunit@v2.3.0...v2.4.0)

---
updated-dependencies:
- dependency-name: phpspec/prophecy-phpunit
  dependency-version: 2.4.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]