greg.apps: fix ugettext_lazy deprecation warning

ugettext_lazy has been deprecated in favour of gettext_lazy.
This silences a deprecation warning outputted during test runs.

Fixes: GREG-14

greg/apps.py

 from django.apps import AppConfig
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 
 
 class GregAppConfig(AppConfig):