gnu: rust-toml-0.5: Update to 0.5.6.

* gnu/packages/crates-io.scm (rust-toml-0.5): Update to 0.5.6.
[arguments]: Skip build. Add rust-indexmap-1.0, rust-serde-1.0 to
cargo-inputs. Add rust-serde-derive-1.0, rust-serde-json-1.0 to
cargo-development-inputs.
[properties]: Remove field.

Signed-off-by: Efraim Flashner <efraim@flashner.co.il>