build(deps): 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](rust-random/rand@rand_core-0.5.1...rand_core-0.6.4)

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

Signed-off-by: dependabot[bot] <[email protected]>

Cargo.toml

@@ -22,7 +22,7 @@ rustc-serialize = "0.3"
 newtype_derive = "0.1"
 custom_derive = "0.1"
 rand = "0.7.3"
-rand_core = "0.5.1"
+rand_core = "0.6.4"
 serde = "1.0"
 serde_derive = "1.0"
 serde_json = "1.0"