Replace force_unicode with force_text

Django changed the name to force_unicode and it's only available in
python2 as an alias of force_text. The new name is available since
Django 1.4.2

Closes-Bug: #1345642
Change-Id: Ida8c545ceec7c31999f2497d540a0dc5a653d286
Merge "Replace force_unicode with force_text"