Correct wrong module name for Postgres in example configs

[lutoma]

The example configs list psycopg2 as the name for the Django postgres database module, but it's actually postgresql_psycopg2: https://github.com/django/django/tree/master/django/db/backends/postgresql_psycopg2

[coveralls]

Coverage remained the same at 79.368% when pulling 04cbf4f on lutoma:conffix into 4f4e585 on pretix:master.

[raphaelm]

Guilty as charged, I'm a MariaDB user most of the time... Thanks!