I had the same issue. Fixed by adding line with sed.
dist: xenial
group: edge
before_install:
- sudo apt-get update
- sudo apt-get --yes remove postgresql\*
- sudo apt-get install -y postgresql-11 postgresql-client-11
- sudo sed -i 's/port = 5433/port = 5432/' /etc/postgresql/11/main/postgresql.conf
- sudo cp /etc/postgresql/{9.6,11}/main/pg_hba.conf
- sudo service postgresql restart 11