GDK needs its own .ruby-version

Overview

Each Ruby update for GitLab breaks GDK because the GDK currently relies upon GitLab's .ruby-version and so when GitLab updates its .ruby-version, the GDK also gets updated.

Proposal

Instead of symlinking GDK_ROOT/gitlab/.ruby-version as GDK_ROOT/.ruby-version, we should have a .ruby-version checked in and managed.