Fix u64 deserialization

[litchipi]

Past issue: https://github.com/toml-rs/toml-rs/issues/256

u64 cannot be deserialized as it's backed by a i64 format

This causes this to fail:

let data = format!("id = {}", u64::MAX);
let val : toml::Value = toml::from_str(&data).unwrap();
println!("{:?}", val);

And any number > i64::MAX will also fail.