Bump failure from 0.1.5 to 0.1.6

Bumps [failure](https://github.com/rust-lang-nursery/failure) from 0.1.5 to 0.1.6.
- [Release notes](https://github.com/rust-lang-nursery/failure/releases)
- [Changelog](https://github.com/rust-lang-nursery/failure/blob/master/RELEASES.md)
- [Commits](https://github.com/rust-lang-nursery/failure/commits)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>