Bump puppetlabs/postgres module to 2.3.0.

I got errors when I tried to use 2.0.1 on Fedora 18. Upgrading
seems to have fixed the issues.

Change-Id: I90fbf45a90e42481f48643e7fca91ba105470a7a
Reviewed-on: https://review.openstack.org/32450
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins