fix fedmsg/relay for DNF (F>21)

nothing that freezes uses this, so I'm OK to send it, according
to nirik.

roles/fedmsg/relay/tasks/main.yml

@@ -1,11 +1,19 @@
 # This is a *very* simple role.  The config needed for fedmsg-relay to operate
 # correctly is actually included as part of the fedmsg/base role.
 
-- name: install fedmsg-relay
+- name: install fedmsg-relay (yum)
   yum: pkg=fedmsg-relay state=present
   tags:
   - packages
   - fedmsg/relay
+  when: ansible_distribution_major_version|int < 22
+
+- name: install fedmsg-relay (dnf)
+  dnf: pkg=fedmsg-relay state=present
+  tags:
+  - packages
+  - fedmsg/relay
+  when: ansible_distribution_major_version|int > 21
 
 - name: ensure that nrpe has rights to monitor us
   file: >