gitlab-shell doesn't work on Ruby v2.0

This Ruby version is used on some macs, so we see this in development:

`require': /Users/james/Development/gdk/go-gitlab-shell/src/gitlab.com/gitlab-org/gitlab-shell/lib/gitlab_shell.rb:185: syntax error, unexpected ',' (SyntaxError)
  def exec_cmd(executable, gitaly_address:, token:, json_args:)

It works in Ruby 2.1, but not 2.0

We're rewriting all of gitlab-shell into go to avoid these kinds of problems, but in the meantime, is it worthwhile ensuring that gitlab-shell works with all versions of ruby we're likely to see in the wild? For me, that's 1.9.3+

cc @rspeicher @DouweM @jameslopez