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.
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