As some of you might have noticed, rubygem-rkerberos is currently not installable on EL 7.5, as it depends on an older libkadm5clnt_mit.so.8 that is no longer part of EL 7.5.
This is tracked upstream in EPEL BZ #1563125 and there is an update (FEDORA-EPEL-2018-90207015c0) available for testing.
We are tracking this in #23241, you can follow the redmine issue for updates.
If you are running Foreman and want to upgrade the base OS to EL 7.5, you can either hold off updating libkadm5 (and all the related Kerberos packages) or try the build from the update above and vote for it on the Fedora Updates System.