(maint) Use Artifactory as gem source
(maint) Use Artifactory as gem source

This commit updates the Gemfile source to Artifactory, rather than rubygems.org.
For some reason, bundle install is failing when using rubygems (only for this
project as far as I can tell). Since Artifactory mirrors upstream rubygems,
this fixes the issue.

puppetlabs/puppetserver,puppetlabs/puppetserver,puppetlabs/puppetserver,puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppet-server,puppetlabs/puppet-server