Ruby > Hamming: Iteration #2

Some of the improvements I made here:

- Changes 'strand' variable names from 'a' and 'b' to 'first_strand' and
  'second_strand' which is much more descriptive.

- 'splat' the arguments coming in so we don't have to keep passing around
  'first_strand' and 'second_strand'

- Use 'count' instead of 'inject' as it makes the code cleaner,
  especially since we were essentially just getting the 'count' anyway.

- Uses 'take_while' after the 'zip' to eliminate the need to check for
  'nil' on the second character of the nucleotide pair.

- Changed 'did_pair_mutate' to 'mutation?' which is more Ruby like and
  feels clearer.

- No longer explicitly returns 'true' in the 'mutation?' method, that
  was unnecessary.