Add sum_squares_lo and sum_squares_hi.

These two fields were not part of the initial spec, so we add them now and
increment the payload's "ver" field accordingly.
Don't bother incrementing the payload version.

Since we're still in pre-release, we don't really need to increment the payload
version yet.
Merge pull request #4 from mreid-moz/add_sum_squares_lo_hi

Add sum_squares_lo and sum_squares_hi.