Bumps [strong_migrations](https://github.com/ankane/strong_migrations) from 0.6.8 to 0.7.1.
- [Release notes](https://github.com/ankane/strong_migrations/releases)
- [Changelog](https://github.com/ankane/strong_migrations/blob/master/CHANGELOG.md)
- [Commits](https://github.com/ankane/strong_migrations/compare/v0.6.8...v0.7.1

)

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