Add workaround to support django 1.5

JSON serialization is different in Django 1.5.

This change does not change anything but the `roughage.serializers.Serializer`. If it's django 1.5, it "rollsback" last changes in `start_serialization()` and `end_serialization()`.

Another alternative is to inherit from a new serializer class, that behaves like Django 1.4 JSON Serializer.

References:

* https://code.djangoproject.com/ticket/5423
* https://github.com/django/django/commit/3b5083b


Can you please review, @spulec?