Do not apply fix for multichoice help text in later version of Django

The initial fix could cause problems in Django 1.7 when serializers get used as a field. This is due to `force_text` being applied before full initialisation of the apps registry (see #1907).

The fix doesn't seem to be required after version 1.6:
https://code.djangoproject.com/ticket/9321

Reg this PR:
I wasn't able to write a meaningful test for this since I'm not sure how to properly replicate in a test environment.