Bump symfony/swiftmailer-bundle from 3.2.6 to 3.2.8

Bumps [symfony/swiftmailer-bundle](https://github.com/symfony/swiftmailer-bundle) from 3.2.6 to 3.2.8.
- [Release notes](https://github.com/symfony/swiftmailer-bundle/releases)
- [Changelog](https://github.com/symfony/swiftmailer-bundle/blob/master/CHANGELOG)
- [Commits](symfony/swiftmailer-bundle@v3.2.6...v3.2.8)

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