Drop CI testing of PostgreSQL 9.2 for 9.6

Now that 10.0 is out, should we just test with the 9.6 image?

Ideally we'd test all our specs against 9.2 and 9.6 for now (maybe only in a nightly build?), but that would be overkill for every branch.

The following discussion from !3136 (merged) should be addressed:

• @rymai started a discussion: (+3 comments)

  I'm wondering if there's a reason why we don't use this image instead of postgres:9.2 in .use-pg directly? Omnibus is using PG 9.6.3 and our own documentation states:

  We highly recommend users to use at least PostgreSQL 9.6 as this is the PostgreSQL version used for development and testing.

  – https://docs.gitlab.com/ce/install/requirements.html#postgresql-requirements