Update the license to match LICENSE file.

Transforms use a target version for names to support uniform versioning.

There were a missing parenthesis in some class definition.

[docs] Fix typo in code samples

* Remove python 3.3 testing - no longer on travis.

* Fix deprecation error in drf-3.6.

* Allow mixins with plain Serializer.

For example:

```
class FooSerializerV3(BaseVersioningSerializer, serializers.Serializer):
    test_field_two = serializers.CharField()

class TestSerializerV3(BaseVersioningSerializer, serializers.ModelSerializer):
    transform_base = 'tests.test_transforms.TestModelTransform'

    class Meta:
        model = TestModelV3
        fields = (
            'test_field_two',
            'test_field_three',
            'test_field_four',
            'test_field_five',
            'new_test_field',
            'new_related_object_id_list',
        )
```