added django.contrib.postgres to installed apps for tests

Author: MartinCervenkaSB

tests/settings.py

@@ -20,6 +20,7 @@
     "django.contrib.admin",
     "django.contrib.auth",
     "django.contrib.contenttypes",
+    "django.contrib.postgres",
     "django.contrib.sessions",
     "django.contrib.messages",
     "easy_thumbnails",