Ruby Proverb added test to avoid someone hardcoding the required lyrics.

[stevenwu's solution](http://exercism.io/submissions/fadc475266c6eb665a3910d7) was to hard code a hash with all the known key/value pairs. So if there were any new items in the proverb his program wouldn't cope. Whereas a standard programmed solution would. I've added a test to stop this being a viable solution.

We must stop this *crushes paper cup for emphasis* hehe ;-)

All tests pass on the example.rb solution and my own solution.
test_proverb_does_not_hard_code_the_rhyme_dictionary *fails* when the code can't cope with an unknown key/value combination.

Rich