Add rubygem-devel as explicit requirement for 13.1

It seems that the handling of Ruby gems for openSUSE 13.1 was changed. A
backport repository is automatically used and ruby-devel needs to be
required explicitly.
Merge pull request #260 from SUSE/add_ruby-devel_as_13.1_requirement

Add rubygem-devel as explicit requirement for 13.1