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

https://en.opensuse.org/openSUSE:Packaging_Ruby#Format_of_automatically_generated_Provides:_headers
Don't specify the ruby ABI; this layer need not care.

any Ruby will do, as long as the lower layers work
Merge pull request #104 from yast/rubygem-requires

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