Make sure the release number compares higher than the previous builds
    
    It went from
    0.2013.03.26.0 (F19)
    to
    0.2014.03.26.0 (F20)
    to
    0.1.2013.08.14.0 (F20)
    
    Unfortunately, 0.2013 and 0.2014 compare higher than 0.1, so the release
    number was actually going backwards.
    
    https://lists.fedoraproject.org/pipermail/devel/2013-October/189917.html
    
    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.