fix travis build for postgres

Author: Gagaro

.travis.yml

@@ -6,6 +6,12 @@ python:
   - 3.5
   - 3.6
 
+addons:
+  postgresql: 9.6
+  apt:
+    packages:
+    - postgresql-9.6-postgis-2.3
+
 env:
   matrix:
   - DJANGO_VERSION=1.6.11 DATABASE=postgres