Make sure the release number compares higher than the previous builds
It went from
Unfortunately, 0.2013 and 0.2014 compare higher than 0.1, so the release
number was actually going backwards.
Also, while at this, I've moved the snapshot_date to a %define so that
rpmdev-bumpspec wouldn't try to bump 2013 to 2014, like happened during
the F20 mass rebuild.