Bump futures from 0.1.20 to 0.2.1

Bumps [futures](https://github.com/rust-lang-nursery/futures-rs) from 0.1.20 to 0.2.1.
- [Release notes](https://github.com/rust-lang-nursery/futures-rs/releases)
- [Changelog](https://github.com/rust-lang-nursery/futures-rs/blob/master/CHANGELOG.md)
- [Commits](rust-lang/futures-rs@0.1.20...0.2.1)

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

Author: dependabot-support