Allow Ruby upgrades to not impact the gdk command line tool

Overview

I'd really like to remove the gitlab-development-kit gem, for the following reasons:

1. Every time a Ruby update occurs, the gdk tool ceases to work.
2. Users very rarely gem update gitlab-development-kit so it's really hard to get needed updates onto user's systems.
3. The main logic within the gem is tasked with the following and can easily be moved into the lib/gdk.rb or such:
   • Initialising a new GDK instance - I'd like to eventually/shortly replace with a good old git clone https://gitlab.com/gitlab-org/gitlab-development-kit.git
   • Trusting the new install - I feel we can gain 'trust' by inspecting the current working directory and looking for the .gdk-install-root rather than having to maintain another file, outside of the GDK.root
   • Remembering that an install took place - This logic can easily be moved into gdk install.

Proposal

Instead of gdk being a Ruby shim, we should make it a Shell script, that lives within /usr/local/bin that is installed/kept up-to-date via a gdk install/update and mostly serves to forward execution over to the GDK in the current working directory.

We'd need to be careful of the situation where most users will then have /usr/local/bin/gdk and a gdk Ruby shim. We should handle this as part of the management of /usr/local/bin/gdk to ask the user to uninstall the gitlab-development-kit Ruby gem.

Once the use of /usr/local/bin/gdk is widely rolled out, we should publish a final gitlab-development-kit Ruby gem that advises the gem is no longer required and change it's logic to simply exec('/usr/local/bin/gdk').

#!/bin/sh

if [ -f ./lib/gdk.rb ]; then
  /usr/bin/env ruby ./lib/gdk.rb $*
else
  echo "ERROR: The current working directory is not inside a gitlab-development-kit installation" >&2
  exit 1
fi

cc @toon, @tigerwnz, @leipert, @deuley