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

https://en.opensuse.org/openSUSE:Packaging_Ruby#Format_of_automatically_generated_Provides:_headers
Merge pull request #57 from yast/rubygem-requires

Use a more flexible rubygem requirement syntax (bnc#895069)
Merge branch 'master' of github.com:yast/yast-devtools into create_maintenance_branch_script

Conflicts:
	package/yast2-devtools.changes