chapter 20 - remove FnBox workaround

As in Rust 1.35.0, FnBox is no loger needed.
https://blog.rust-lang.org/2019/05/23/Rust-1.35.0.html

It is perfectly OK to call `job();` now

```rust
match message {
    Message::NewJob(job) => {
        job();
    }
```

Is there any plan to remove the FnBox workaround?