Use a more flexible rubygem requirement syntax (bnc#895069)

Oh. I missed those :( @darix?

Additionally, we have:
http://river.suse.de/view/YaST/job/yast-services-manager-master/42/console
```
unresolvable: have choice for rubygem(yast-rake) >= 0.1.7: rubygem-yast-rake ruby2.1-rubygem-yast-rake
```