Update frunk_core requirement from 0.3.0 to 0.4.0

Updates the requirements on [frunk_core](https://github.com/lloydmeta/frunk) to permit the latest version.
- [Release notes](https://github.com/lloydmeta/frunk/releases)
- [Changelog](https://github.com/lloydmeta/frunk/blob/master/CHANGELOG.md)
- [Commits](lloydmeta/frunk@v0.3.0...0.4.0)

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

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

Cargo.toml

@@ -40,7 +40,7 @@ futures = "0.3"
 # Conversion
 frunk = { version = "0.3.0", optional = true }
 frunk_derives = { version = "0.3.0", optional = true }
-frunk_core = { version = "0.3.0", optional = true }
+frunk_core = { version = "0.4.0", optional = true }
 frunk-enum-derive = { version = "0.2.1", optional = true }
 frunk-enum-core = { version = "0.2.1", optional = true }