Train on upgrading 9.0 to PostgreSQL 9.6

The build team has been doing a great job trying to be thorough and cover every potential failure with this, but something tells me the first issue many people will hit in upgrading to GitLab 9.0 is the automatic upgrade to PostgreSQL 9.6.

For users not using a bundled PostgreSQL server (e.g. RDS), I don't think this is as a big of an issue.

I recommend:

• Take one of your dev instances with omnibus and attempt to upgrade to the latest 9.0 RC.
• Learn the commands necessary to test that the right version is installed.
• See if you can find an instance where the upgrade fails.
• Learn how to recover from the failure (e.g. data gets moved to /var/log/gitlab/postgresql/data.9.2.18) and initiate the upgrade again

We're still working out how this affects GitLab Geo, but I documented what I think so far here: https://gitlab.com/gitlab-org/gitlab-ee/issues/1917#note_25550364

/cc: @lbot