Add rb-readline package

...as a dependency of https://github.com/theforeman/hammer-cli/pull/127

Scratchbuilds:
- http://koji.katello.org/koji/taskinfo?taskID=147821
- http://koji.katello.org/koji/taskinfo?taskID=147823
- http://koji.katello.org/koji/taskinfo?taskID=147824
%gem_install is preferable nowadays