Fix bug in checking MIDDLEWARE setting on Django 1.11

Author: daneah

.travis.yml

@@ -4,6 +4,8 @@ python:
   - "3.5"
   - "3.6"
 env:
+  - DJANGO_VERSION=1.8.18
+  - DJANGO_VERSION=1.9.13
   - DJANGO_VERSION=1.10.8
   - DJANGO_VERSION=1.11.9
   - DJANGO_VERSION=2.0.1

webmention/checks.py

@@ -8,7 +8,7 @@ def new_style_middleware_check(app_configs, **kwargs):
     errors = []
 
     if django.VERSION[1] >= 10 or django.VERSION[0] > 1:
-        installed_middlewares = getattr(settings, 'MIDDLEWARE', [])
+        installed_middlewares = getattr(settings, 'MIDDLEWARE', []) or []
         if 'webmention.middleware.WebMentionMiddleware' in installed_middlewares:
             errors.append(
                 Error(