Ensure INSTALLED_APPS sanity for flat app according to Django version

flat/__init__.py

@@ -1 +1,2 @@
 __version__ = '1.1.3'
+default_app_config = 'flat.apps.FlatConfig'

flat/apps.py

@@ -0,0 +1,13 @@
+# -*- coding: utf-8 -*-
+
+from django.apps import AppConfig
+
+
+class FlatConfig(AppConfig):
+    name = 'flat'
+    verbose_name = 'Django flat theme'
+
+    def ready(self):
+        from flat import settings
+
+        settings.check_installed_apps()

flat/settings.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+
+import django
+from django.conf import settings
+from django.core.exceptions import ImproperlyConfigured
+
+
+def check_installed_apps():
+    dj_version = django.VERSION
+    installed_apps = settings.INSTALLED_APPS
+
+    if dj_version < (1, 9):
+        if 'flat' not in installed_apps:
+            raise ImproperlyConfigured(
+                '\'flat\' needed before django 1.9, '
+                'add it to settings.INSTALLED_APPS.')
+    else:
+        if 'flat' in installed_apps:
+            raise ImproperlyConfigured(
+                '\'flat\' not needed since django 1.9, '
+                'remove it from settings.INSTALLED_APPS.')