Add PostgreSQL 10 to Geo CI Build

Fixes were made (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4669) to allow for Geo to run on PostgreSQL 10. We should add a CI step to start exercising it.

Start by adding a Geo-specific test for PostgreSQL 10 as we do with with the .rspec-geo-pg-9-6 entry.

See https://gitlab.com/gitlab-org/gitlab-ee/blob/master/.gitlab-ci.yml#L201 and https://gitlab.com/gitlab-org/gitlab-ee/blob/master/.gitlab-ci.yml#L476.