create a VerseTwoStrategy to make the '2 bottles' spec pass

While this makes the specs pass, it needs refactoring. We are passing
an unused num argument to VerseTwoStrategy#verse(num). And I'm not sure
I'm confident in the way the way the verse strategy factory method
works.
refactor strategy classes to use class methods

I'm not sure I like this. I generally prefer instance methods to class
methods. But in this case, it seems a little cleaner than the last
commit.