Refactors of_a_kind

- Removes the conditional and the `results` temporary variable
- Also renames the block variables in `of_a_kind`, `raw_frequency`, and `order` to give a better idea of what they represent
Adds sum method

Moves the contents of the `chance` method into the `sum` method. Also uses `sum` in `full_house`.
Refactors valid_roll?

Breaks up into `valid_size?` and `valid_values?` helper methods for readability.
Adds a few more tests for custom errors
Updates Rakefile