Update Postgres to 9.6 for CentOS in docs
Bump from 9.5 to PostgreSQL 9.6 for CentOS in the documentation (doc/prepare.md).
CI of gitlab-ce/gitlab-ee uses 9.2 (gitlab-org/gitlab-ce!16277), Geo uses 9.6, and Omnibus ships with 9.6.
cf.
- #292 (closed) (GDK's Dockerfile)