Bump rand_chacha from 0.1.1 to 0.3.1 in /rust

Bumps [rand_chacha](https://github.com/rust-random/rand) from 0.1.1 to 0.3.1.
- [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-0.1.1...rand_chacha-0.3.1)

---
updated-dependencies:
- dependency-name: rand_chacha
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

rust/Cargo.lock

@@ -53,4 +53,4 @@ opt-level = "s"
 quickcheck = "0.9.2"
 quickcheck_macros = "0.9.1"
 rand_os = "0.1"
-rand_chacha = "0.1"
+rand_chacha = "0.3"