Review Board 3.0.2 introduced a revised binary format for diff fragments

and review request page entry updates, which uses Python's struct to

pack numbers into bytes, given a packing rule string. Python 2.7.7 and

higher allow Unicode strings here, but older versions require byte

strings. This means that on old Python installs, diff fragments and

entry updates were broken for users.
This change simply switches those to using byte strings, ensuring the

widest compatibility. These work on all versions of Python 2.x and 3.x.

Unit tests pass.
Customer verified this fix on Python 2.7.5.