Fedora doesn't use YUM anymore, should migrate to DNF

[XVilka]

For many releases DNF is the default package manager, maybe it makes sense to update it from Yum to DNF? It will require dropping some older versions though (seems only Fedora 21 should be removed then).

https://fedoraproject.org/wiki/Releases/22/ChangeSet#Replace_Yum_With_DNF

[avsm]

PR welcome for this.