Travis: Give up trying to use PostgreSQL 11

.travis.yml

@@ -39,17 +39,15 @@ script:
   - tox -e $TOXENV
 
 addons:
-  # test with last version that still supports OIDs
-  postgresql: "11"
+  # better test with latest PostgreSQL version (13)
+  # or last version that still supports OIDs (11)
+  # or ideally both, but difficult to configure in Travis
+  # because of conflicts with the default PostgreSQL
+  postgresql: "10"
 
 services:
   - postgresql
 
 before_script:
-  - sudo service postgresql stop
-  - sudo apt-get remove -y postgresql\*
-  - sudo apt-get install postgresql-11 postgresql-client-11
-  - sudo -u postgres sed -i "s/port = 54[0-9][0-9]/port = 5432/" /etc/postgresql/11/main/postgresql.conf
-  - sudo service postgresql start 11
   - sudo -u postgres psql -c 'create database unittest'