I had previously used TOX_ENV, but not realised that TOXENV has a
special meaning for tox. We can now expect TOX to run the appropriate
build on travis without having it explicitly passed into the tox
command.
I'm a little concerned that this loses some of the efficiencies (in
particular, chunking) from previous versions. That's something that can
probably be improved.
This avoids the following error in django 1.11 tests:
polymorphic.MRODerived: (models.E005) The field 'id' from parent model 'polymorphic.mrobase3' clashes with the field 'id' from parent model 'polymorphic.mrobase1'.
Related to https://code.djangoproject.com/ticket/22442