remove usage of simplejson+remove django1.5 checks

Hello.
Since the first page of documentation of django tastypie stat that tastypie rely on django 1.5+ and that django 1.5 stat that usage of django.utils.simplejson is deprecated, I made a commit to remove the use of simplejson. And I also made a second commit for removing checks for django version prior to 1.5, since it's not supposed to be supported, no need to keep promised-to-be-dead code.