Use DNF instead of yum

[AdamWill]

It's unclear whether there's any urgency to this (i.e. whether it's considered 'required' for F22 as part of https://fedoraproject.org/wiki/Changes/ReplaceYumWithDNF ), but it's gonna need to get done at some point, so may as well have a ticket for it.

For everything it currently uses yum for, fedup should use dnf instead, cos it's the new shiny. (As a side note, I believe it also makes #21 more viable).

wwoods posted a high-level assessment of what needs doing to devel@: https://lists.fedoraproject.org/pipermail/devel/2015-January/207152.html

[igor-raits]

I can help with this, but I'm not familiar with fedup architecture.

[AdamWill]

it's not really very complicated. just grep the whole tree for yum and it's only significant in a couple of files.