Use a lockfile for yum stuff.

This will hopefully fix
https://fedorahosted.org/fedoracommunity/ticket/401

If it does, then I owe @lmacken one of these:
http://www.ratebeer.com/beer/brewdog-tokyo/107203/
Merge pull request #70 from fedora-infra/feature/yumlock

Use a lockfile for yum stuff.