Add default DJANGO_SECRET_KEY

.travis.yml

@@ -2,6 +2,10 @@ language: python
 python:
     - "3.5"
     - "3.6"
+
+env:
+    # Set a dummy secret key 
+    - DJANGO_SECRET_KEY=0
 # install dependencies
 install: "pip install -r requirements.txt"
 script: python manage.py test