Refactor to Card::SUITS

Here's an approach for defining `SUITS` that's [more cohesive](https://en.wikipedia.org/wiki/Cohesion_(computer_science)). What do you think?

Also, the `bundle install --path vendor/bundle` command installs all the gems to the `vendor/bundle` directory within the project. I've ignored it and the `.bundle` configuration so you can continue to install gems globally if you want.