Bump futures from 0.1.25 to 0.3.15

Bumps [futures](https://github.com/rust-lang/futures-rs) from 0.1.25 to 0.3.15.
- [Release notes](https://github.com/rust-lang/futures-rs/releases)
- [Changelog](https://github.com/rust-lang/futures-rs/blob/master/CHANGELOG.md)
- [Commits](rust-lang/futures-rs@0.1.25...0.3.15)

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

Author: dependabot-preview[bot]