Remove self._state.adding check

The code for ModelState can be viewed here: https://github.com/django/django/blob/master/django/db/models/base.py#L358

From what I gather, `self._state.adding = True` is the case when you use a proxy model as described in the docs: http://django-parler.readthedocs.org/en/latest/advanced/existing.html

All tests are still green, but this update is not something that I feel 100% on top of :) I can tell, though, that I have both normal translations and proxy model translations running fine after the change.

Remaining bug: One big problem remaining is that if - say - Portuguese is my currently set language, i.e. that I'm in `/pt/admin/app/translatable-model/123/?language=de` and I add or edit the German version, the Portuguese version is also overwritten. I have no idea if this behaviour is linked to the change, but it *only* happens on proxy models. Normal model inheritors of TranslatableModel have not problems.