Activity plugin does not work with primary keys named other than "id"

[dryobates]

SQLAlchemy-Continuum: 1.3.7
SQLAlchemy: 1.2.17

If I name primary_key of my model other than "id", then it raises "AttributeError: type object 'Version' has no attribute 'id'".

Test that shows problem:

class TestActivityNotId(ActivityTestCase):

    def create_models(self):
        TestCase.create_models(self)

        class NotIdModel(self.Model):
            __tablename__ = 'not_id'
            __versioned__ = {
                'base_classes': (self.Model, )
            }

            pk = sa.Column(sa.Integer, autoincrement=True, primary_key=True)
            name = sa.Column(sa.Unicode(255), nullable=False)
        self.NotIdModel = NotIdModel

    def test_create_activity_with_pk(self):
        not_id_model = self.NotIdModel(name="abc")
        self.session.add(not_id_model)
        self.session.commit()
        self.create_activity(not_id_model)
        self.session.commit()
        activity = self.session.query(versioning_manager.activity_cls).first()
        assert activity
        assert activity.transaction_id
        assert activity.object == not_id_model
        assert activity.object_version == not_id_model.versions[-1]