Fixes of_a_kind

The `of_a_kind` method was not counting of-a-kinds that were contained in greater of-a-kinds, e.g., `of_a_kind 2` would return 0 for rolls such [1, 1, 1, 2, 3] and [6, 6, 6, 6, 6].

Detecting `die_count >= kind_count` fixes this. I also added an `if_none` lambda to avoid returning `nil` so we can continue the calculation naturally.
Ensures two_pairs only scores distinct pairs

The rules for KataYahtzee are not explicit on whether rolls with 4 or 5 of the same die value are considered eligible for scoring. Since a kahtzee is explicitly not a full house, I've decided that `two_pairs` will only score distinct pairs — in all other cases, the score for this category will be 0.
Updates README