Replaced custom docker image for PostgreSQL with the official one

See https://travis-ci.org/doctrine/dbal/jobs/488051004 for the dependency issue.

tests/travis/Dockerfile-postgres11

-FROM debian:experimental-20180426
-
-RUN apt-get update && \
-    apt-get install -y -t experimental --no-install-recommends \
-    postgresql-11 \
-    postgresql-client-11 \
-    && \
-    rm -rf /var/lib/apt/lists/*
-
-RUN echo "host all all all trust" >> /etc/postgresql/11/main/pg_hba.conf
-RUN echo "listen_addresses='*'" >> /etc/postgresql/11/main/conf.d/listen.conf
-
-EXPOSE 5432
-
-CMD ["sleep", "inf"]

tests/travis/install-postgres-11.sh

@@ -6,9 +6,7 @@ echo "Preparing Postgres 11"
 
 sudo service postgresql stop || true
 
-sudo docker build -t postgres11 - < tests/travis/Dockerfile-postgres11
-sudo docker run -d --name postgres11 -p 5432:5432 postgres11
-sudo docker exec postgres11 service postgresql start
-sudo docker exec -i postgres11 su -c psql postgres <<<"create database doctrine_tests"
+sudo docker run -d --name postgres11 -p 5432:5432 postgres:11.1
+sudo docker exec -i postgres11 bash <<< 'until pg_isready -U postgres > /dev/null 2>&1 ; do sleep 1; done'
 
 echo "Postgres 11 ready"