fix postgresql version in .travis.yml

.travis.yml

@@ -6,8 +6,9 @@ cache: pip
 python:
   - '3.6'
 
-services:
-  - postgresql
+addons:
+  # Django 2.1+ requires PostgreSQL 9.4+
+  postgresql: '9.4'
 
 install:
   - pip install -r requirements.txt