add needed ruby:backports repo for SLES

Since about two weeks, the package rubygem-hiera from systemsmanagement:puppet
depends on the binary /usr/bin/ruby.ruby1.8 which is not provided by
ruby-1.8.7.p357-0.9.9.1 comming with SLES11 SP3.

ruby-1.8.7.p357-0.9.16.1 from devel:languages:ruby:backports is bringing that
binary and the Puppet agent package can again resolve its dependencies when
doing an unattended install.