Change 'fail!' to 'panic!' in text and code

[yonibot]

In accordance with rust-lang/rfcs#221. This will prevent confusion for new Rust developers who follow the book and get compile errors when 'fail!' is used 😕 .

I left the 'Fail!' text in here because it seemed to be a description of what was happening, rather than a command:

#[test]
fn this_tests_code() {
    panic!("Fail!");
}

[steveklabnik]

hey thanks! Rust for Rubyists tracks releases, not master, but this will need to go in before the next one! And you're correct about 'fail' there.