Add rb-readline package

...as a dependency of https://github.com/theforeman/hammer-cli/pull/127

Scratchbuilds:
 - http://koji.katello.org/koji/taskinfo?taskID=147843
 - http://koji.katello.org/koji/taskinfo?taskID=147844
 - http://koji.katello.org/koji/taskinfo?taskID=147846