Comply with Django>=1.11 assumptions on models.
Comply with Django>=1.11 assumptions on models.

Starting with Django 1.11 (commit
a44fc008c4e38fadef04021fdba564949f1aee3a), Django makes assumptions on
the class hierarchy of a Model subclass:

If one of its bases isn't `django.db.models.Model`, yet is an instance
of `django.db.models.base.ModelBase` (through some metaclass), then it
should have a `._meta` attribute - in other words, it should be a
`django.db.models.Model` subclass as well.

This commit changes our WorkflowEnabled class to inherit from
`django.db.models.Model`, which should fix the current issue.

Closes #30.

rbarrois/django_xworkflows