Remove the code that simulates the different player skill levels

I just didn't feel that that feature needed to be a concern of the
XO::AI module.
Don't track .ruby-gemset and .ruby-version
Put the Ruby version requirement within the gemspec
Update gems
Ensure the grid's helper methods are private
Convert XO::Evaluator into a Singleton class
Apply extract method refactoring step to improve the code's readability