Fix failing PostgreSQL install on travis-ci

Presumably due to a change to the travis base image [1]
the init of a database cluster failed.
Since we don't need a prepared cluster, but the executables only,
this can be solved by the extract-only mode.

[1] https://blog.travis-ci.com/2017-12-12-new-trusty-images-q4-launch

.travis.yml

@@ -36,7 +36,7 @@ before_install:
     if [ -z "$PGCROSS" ]; then
       wget http://get.enterprisedb.com/postgresql/postgresql-$PGVERSION.run && \
         chmod +x postgresql-$PGVERSION.run && \
-        sudo ./postgresql-$PGVERSION.run --mode unattended --unattendedmodeui minimal
+        sudo ./postgresql-$PGVERSION.run --extract-only 1 --mode unattended
     fi
 script: |
   if [ -z "$PGCROSS" ]; then