Refactoring code in RPS branch

@gabebw @jsteiner
You will almost never (say 99.99% of the time) ever compare directly to `true` or `false`. Instead, do this: `while ! is_choice_valid?` or, better: `while is_choice_invalid?`.

Then remove the `is_`, because the question mark at the end does that for you. I've never seen a Ruby method that started with `is_`. So: `while invalid_choice?` is what you end up with, which reads better.

Also: `while` loops don't use `do`.