Fix `agree` prompts

Capital letter (`Y` on `N`) in prompts indicate a default action in
console UIs. `Highline`'s `agree` method does not accept empty input
though, meaning there is no default action:

> Create unicorn configuration file? [Yn]
>
> Please enter "yes" or "no".

This confuses users (well, it confused me!).
Merge pull request #4 from elegion/fix-agree-prompts

Fix `agree` prompts