travis: Update postgreSQL 10 steps

travis: Update postgreSQL 10 steps

This is failing since the update to Xenial [1] with the following
warning:

  $ sudo -u postgres psql -c "grant ALL on DATABASE postgres to travis WITH GRANT OPTION;"
  could not change directory to "/home/travis/build/getpatchwork/patchwork": Permission denied
  psql: could not connect to server: No such file or directory
    Is the server running locally and accepting
    connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5433"?

Use the solution from [2] to resolve it.

[1] https://travis-ci.org/getpatchwork/patchwork/jobs/582342649
[2] https://github.com/travis-ci/travis-ci/issues/8537#issuecomment-498299645

Signed-off-by: Stephen Finucane <stephen@that.guru>