Bump symfony/flex from 2.5.1 to 2.7.0 (#347)

Bumps [symfony/flex](https://github.com/symfony/flex) from 2.5.1 to 2.7.0.
- [Release notes](https://github.com/symfony/flex/releases)
- [Commits](symfony/flex@v2.5.1...v2.7.0)

---
updated-dependencies:
- dependency-name: symfony/flex
  dependency-version: 2.7.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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