ignore silly lambda warning

.travis.yml

@@ -10,7 +10,7 @@ install:
   - pip install -e .
   - pip install -r requirements/v2.txt $DJANGO
 before_script:
-  - flake8 celery_haystack --ignore=E501
+  - flake8 celery_haystack --ignore=E501,E731
 script:
   - coverage run --branch --source=celery_haystack `which django-admin.py` test celery_haystack
   - coverage report --omit=celery_haystack/test*