Rubocop Sample config file and exempted cops in existing code

Take one or take both commits, but something to look at.

I will write something later in the README file to give hints on using Rubocop with exercise submissions.

re: #64 
This happens because the range is being used (line 62>65 in the diff).  If we use parenthesis for the `assert_equal` method then it becomes clear that it is not the flip-flop operator that is being used, or some other group expression.  And the fact that there is no opening parenthesis on the method call is exactly why this lint appears.