Make crypto_utils backwards compatible with python-crypto on CentOS 6.4

CentOS ships an older version of pycrypto by default, with the newer version
only available in EPEL. This change makes Review Board work with either.

Testing done: Was able to get ReviewBoard to work on CentOS 6.4

Reviewed at https://reviews.reviewboard.org/r/4769/
Fixes bug 3110