Update the random number generation page to use rand 0.6

[ivandardi]

The rand crate just released its 0.6 release, so the cookbook should be updated accordingly, especially since the API got a big more ergonomic to use.

Here's an update guide: https://rust-random.github.io/book/update-0.6.html

Here's an overview of how code with rand looks like: https://rust-random.github.io/book/overview.html

The biggest benefit is the random() function being able to generate values of any type with just importing the prelude, which is really useful. One note I'd like to say is that generating random booleans should be in the cookbook because that functionality is extremely common and most people use if rng.gen::<f64>() < 0.5 which works but doesn't look as good as the recent if random()

[dhardy]

Slight correction: random() isn't new; it's had roughly that functionality as long as I remember (perhaps not everyone realised).

We also have e.g. rng.gen_bool(0.2) if you want other probabilities.