Change force_text to force_str to remove RemoveInDjango40Warning

[rhenter]

Change force_text to force_str to remove RemoveInDjango40Warning

.../smart_selects/utils.py:73: RemovedInDjango40Warning: force_text() is deprecated in favor of force_str().
  results.sort(key=lambda x: unicode_sorter(force_text(x)))
.../smart_selects/utils.py:55: RemovedInDjango40Warning: force_text() is deprecated in favor of force_str().
  {'value': item.pk if str(item.pk).isdigit() else str(item.pk), 'display': force_text(item)} for item in results

[manelclos]

Hi @rhenter thanks for the PR. It seems tests are not passing, maybe we need it to conditionally use one function or another depending on the Django version. I'd not like to drop support for Django 1.8 until we release a new major version.

[rhenter]

Make sense! I'm closing this PR

[manelclos]

@rhenter you can reuse this PR and squash commits at the end.