Testing Game and Guess objects

Testing game initializer; word is selected from dictionary, letters are selected from word
Testing guess credentials; player inputs guess, player gets incorrect guess, player gets correct guess
This test is a little weak. You should be very explicit in your tests about what you expect to happen. Currently, you are testing that the length of `words` and `letters`, both internal state of `game` are the same length. But this doesn't ensure that it is actually the correct characters, so it could just be an array of all `nil` values for all we know.

Instead of hardcoding the `WordFile` in the `Game` class, I would invert control, and use dependency injection. Basically, that just means pass the word in as an argument to `Game.new`. That way, `Game` doesn't depend on WordFile, it just expects to be passed a word. This will make testing much cleaner:

```
it "returns the word split into characters" do
   word = "wombat"
   game = Game.new(word)

   expect(game.letters).to eq %w(w o m b a t)
end
```

Now we know that `letters` returns the word split into individual characters.