Don't use spelling extension on read the docs

docs/conf.py

@@ -50,9 +50,11 @@ os.environ['DATABASE_ENGINE'] = 'django.db.backends.sqlite3'
 # ones.
 extensions = [
     'sphinx.ext.autodoc',
-    'sphinxcontrib.spelling',
 ]
 
+if not on_rtd:
+    extensions.append('sphinxcontrib.spelling')
+
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']