Add django-reset to dependencies.

We still use 'manage.py reset' in a few cases, and 'manage.py flush' is not an
equivalent replacement. This change pulls in the 'django-reset' package as an
optional dependency, which forward-ports this management command.

Testing done:
Ran './reviewboard/manage.py --help' and saw the reset commands.

Reviewed at https://reviews.reviewboard.org/r/6104/