Bump futures from 0.3.12 to 0.3.13 (#493)

Bumps [futures](https://github.com/rust-lang-nursery/futures-rs) from 0.3.12 to 0.3.13.
- [Release notes](https://github.com/rust-lang-nursery/futures-rs/releases)
- [Changelog](https://github.com/rust-lang/futures-rs/blob/master/CHANGELOG.md)
- [Commits](rust-lang/futures-rs@0.3.12...0.3.13)

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