tox: Update versions of Django to be tested

tox: Update versions of Django to be tested

Since patchwork now supports Django 1.8 and no longer support 1.5, it
is necessary to update the tox targets to validate this new subset of
supported Django versions.

In addition, the paths to the requirements.txt files were not updated
as part of a rebase. Correct this oversight.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>