Travis: Add PG 9.6 to the test matrix

.travis.yml

@@ -11,6 +11,7 @@ env:
     - POSTGIS_VERSION="2.5"
 
   matrix:
+    - POSTGRESQL_VERSION="9.6"
     - POSTGRESQL_VERSION="10"
     - POSTGRESQL_VERSION="11"
 
@@ -19,6 +20,7 @@ before_install:
   - sudo service postgresql stop;
   - sudo apt-get remove postgresql* -y
   - sudo apt-get install -y --allow-unauthenticated --no-install-recommends --no-install-suggests postgresql-$POSTGRESQL_VERSION postgresql-client-$POSTGRESQL_VERSION postgresql-server-dev-$POSTGRESQL_VERSION postgresql-common
+  - if [[ $POSTGRESQL_VERSION == '9.6' ]]; then sudo apt-get install -y postgresql-contrib-9.6; fi;
   - sudo apt-get install -y --allow-unauthenticated postgresql-$POSTGRESQL_VERSION-postgis-$POSTGIS_VERSION postgresql-$POSTGRESQL_VERSION-postgis-$POSTGIS_VERSION-scripts postgis postgresql-plpython-$POSTGRESQL_VERSION
   - sudo pg_dropcluster --stop $POSTGRESQL_VERSION main
   - sudo rm -rf /etc/postgresql/$POSTGRESQL_VERSION /var/lib/postgresql/$POSTGRESQL_VERSION