Using force_text for indexing message

`verbose_name_plural` can be a functional proxy object as it is possible to use `ugettext_lazy` for it, if you wouldn't use `force_text`, the output would be:

    All documents removed.
    Indexing 203 <django.utils.functional.__proxy__ object at 0x30aed10>.
    Indexing 19 <django.utils.functional.__proxy__ object at 0x30a1a50>.
    Indexing 999 <django.utils.functional.__proxy__ object at 0x332f650>.