fix smart_text import (django-rest-framework 3.x)

since code is supposed to work on django >= 1.5, I guess some cleanup
happened on `django-rest-framework`; `rest_framework.compat` does not
expose `smart_text` anymore on the 3.x / master branch.
add drf 3.0.1 to tox.ini
Merge pull request #175 from dnozay/drf-3x

fix smart_text import (django-rest-framework 3.x)