mod_acl_simple_roles: Fix bug where visible_for would be reset when it was not part of a rsc update call.

When visible_for is not given in the update props, we now leave the
value alone, instead of replacing it with the min_visible_for setting,
which essentially overrode any set value in the database.
Update zotonic_install

Installation using one-line script for Debian uses the default postgresql database server (works with lenny and wheezy now).
Merge pull request #575 from carlopires/master

Fix to on-line script installation for Debian