Update rlua requirement from 0.15.0 to 0.19.4

Updates the requirements on [rlua](https://github.com/amethyst/rlua) to permit the latest version.
- [Release notes](https://github.com/amethyst/rlua/releases)
- [Changelog](https://github.com/amethyst/rlua/blob/master/CHANGELOG.md)
- [Commits](mlua-rs/rlua@0.15.0...v0.19.4)

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

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

Author: dependabot[bot]

Cargo.lock

@@ -29,7 +29,7 @@ objpool = "0.2.0"
 packet = "0.1.2"
 parsip = "0.1.0"
 pool = "0.1.4"
-rlua = "0.15.0"
+rlua = "0.19.4"
 serde = "1.0.66"
 serde_derive = "1.0.66"
 time = "0.1.40"