ceph.spec.in: junit always except for EPEL 6

ceph.spec.in: junit always except for EPEL 6

The package was renamed a long time ago (around the Fedora 15
timeframe). The "junit4" name is only relevant for EPEL 6. For EPEL 7
and Fedora 20, the "junit" package has "Provides: junit4". And most
recently, in the junit package that ships in Fedora 21 and 22, the
package maintainer dropped the old Provides: line.

http://tracker.ceph.com/issues/10728 Fixes: #10728

Signed-off-by: Ken Dreyer <kdreyer@redhat.com>
Signed-off-by: Loic Dachary <ldachary@redhat.com>