Add rubygems package dependency.

The dashboard server requires the rubygems package in able to
use "gem" to install rack, rake and rdoc

Change-Id: I442d376eb5ab5aba3b1a17ad999b136885fb4958
Reviewed-on: https://review.openstack.org/23056
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins