Give CentOS / RHEL a chance at installing PostgreSQL 9.0

Give CentOS / RHEL a chance at installing PostgreSQL 9.0

Evergreen 2.1 requires PostgreSQL 9.0, and we accordingly updated the
package names back in the summer to point at PostgreSQL 9.0, but nobody
running CentOS / RHEL brought to our attention that the repo target
needed to be updated as well. This, although untested, at least gives
CentOS / RHEL users a fighting chance of installing Evergreen master /
2.1 on their system.

Signed-off-by: Dan Scott <dscott@laurentian.ca>