Update rand_core requirement from 0.5.1 to 0.6.4

Updates the requirements on [rand_core](https://github.com/rust-random/rand) to permit the latest version.
- [Release notes](https://github.com/rust-random/rand/releases)
- [Changelog](https://github.com/rust-random/rand/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rust-random/rand/compare/rand_core-0.5.1...0.6.4)

---
updated-dependencies:
- dependency-name: rand_core
  dependency-type: direct:production
...

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

Cargo.toml

@@ -20,7 +20,7 @@ features = ["nightly"]
 
 [dependencies]
 curve25519-dalek = { version = "3.0.0", features = ["serde"] }
-rand_core = "0.5.1"
+rand_core = "0.6.4"
 serde = { version = "1.0", features = ["derive"] }
 zeroize = "1.1.0"