Bump rand_chacha from 0.2.1 to 0.2.2

Bumps [rand_chacha](https://github.com/rust-random/rand) from 0.2.1 to 0.2.2.
- [Release notes](https://github.com/rust-random/rand/releases)
- [Changelog](https://github.com/rust-random/rand/blob/master/CHANGELOG.md)
- [Commits](rust-random/rand@rand_chacha-0.2.1...rand_chacha-0.2.2)

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