Quick refactor

I have made this quick refactor to fix some indentation in the files, to require a Ruby version, or just to simplify a bit the code.

If you like it you can accept it :) And, please, test it in your bank account! I don't have Bankinter.

In the next one I'll try to add another bank account. With two bank classes we'll be able to see in a clearer way how to extract those clases in a super-class.