(packaging) Update buildrequires for rhel7

The systemd-units package is not available on rhel7, so this commit moves rhel7
and later into the same BuildRequires as f18 and later.
Merge pull request #163 from haus/maint/2.4.x/rhel7_fix

(packaging) Update buildrequires for rhel7
Merge branch '2.4.x'