Introduce ambiguity in Hamming problem.

This removes the tests that define behavior for cases where the strings are of unequal
length. Since the hamming problem is the first one on the Ruby track, and so many people
who are new to programming start with the Ruby track, I didn't want to introduce error
handling here.

The wikipedia article about the Hamming distance says that the behavior is undefined if
the strings are of unequal length. I don't necessarily think that this means that a particular
implementation _shouldn't_ specify an error, only that it's OK to leave it undefined.

This will also make it possible to bring up the idea of edge cases and error handling in the
discussions on the site.

Closes #44 and #31.