build(deps): bump rand_core from 0.6.3 to 0.6.4 (#378)

Bumps [rand_core](https://github.com/rust-random/rand) from 0.6.3 to 0.6.4.
- [Changelog](https://github.com/rust-random/rand/blob/master/CHANGELOG.md)
- [Commits](rust-random/rand@rand_core-0.6.3...0.6.4)

---
updated-dependencies:
- dependency-name: rand_core
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Cargo.lock

@@ -35,7 +35,7 @@ x25519-dalek = { version = "2.0.0", features = [
     "reusable_secrets",
     "static_secrets",
 ] }
-rand_core = { version = "0.6.3", features = ["getrandom"] }
+rand_core = { version = "0.6.4", features = ["getrandom"] }
 chacha20poly1305 = "0.10.0-pre.1"
 aead = "0.5.0-pre.2"
 blake2 = "0.10"