Updated PostgreSQL version in "Preparing your computing environment for GDK" guide
requested to merge gitlab_performance_test/gitlab-development-kit:503-ni-update-postgresql-version-in-guide into master
Please code review these changes in "Preparing your computing environment for GDK" guide:
PostgreSQL version was updated from 9.6 and 9.x to 10.x according to the issue below.
Closes #503 (closed)