Update postgres to use official postgresql.org apt repository and support postgres 9.3

[fpoms]

http://wiki.postgresql.org/wiki/Apt

postgresql.org now provides an APT repository for PostgreSQL packages for Debian and Ubuntu. This repository has the latest postgres packages, including version 9.3 which is not currently supported by this crate.

The crate should use this official repository and provide support for postgresql version 9.3.