tz fix in openendedchild

uses django.utils.timezone.UTC object that needs to be instantiated to be used with datetime.now. See line 129:https://github.com/edx/edx-platform/blob/d7d115df67a16d222622d2e256d630ce119e3f1a/common/lib/xmodule/xmodule/open_ended_grading_classes/openendedchild.py#L129

@dmitchell 
@dianakhuang