Support PG 9.6 when it comes out

[drnic]

https://www.postgresql.org/message-id/27572.1474662715@sss.pgh.pa.us

This will require a separate docker image, a separate service ID, and "clone-from" will need to ensure the requested service ID matches the backup's service ID (that is, same postgresql version).

[drnic]

In progress. Currently blocked by upgrading concourse to v2.2.1 to fix issues in pipelines relating to docker images.