.travis.yml: Use PostgreSQL 9.5

Default PostgreSQL in Travis CI is 9.2 which is very old, so let's
try to use 9.5.

See: https://docs.travis-ci.com/user/database-setup/#postgresql

.travis.yml

@@ -3,8 +3,8 @@ python:
   - "3.5"
   - "3.6"
   - "3.7-dev"
-sudo: required
-services: postgresql
+addons:
+  postgresql: "9.5"
 before_script:
   - psql --version
   - createuser -U postgres dspacestatistics