Two things were improved:

  - We weren't installing any Django version because the requirements
    file are used by tox and django must be left unspecified so tox can
    cycle through the supported versions.

    That's not really developer friendly though, we're creating yet
    another to get started with patchwork, with a choice to make. We can
    make a good default choice, leaving the developer wanting to
    override the Django version if needed. This way, the common case
    doesn't have an extra step and the not-so-common case is still the
    same number of steps.

  - Support both mysql and postgres DB backends

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>