sos: requring sos >= 3.1 also on F19

sos 3 has been backported to F19 causing log-collector
conflicts as previously on F20.

Change-Id: I6108d9e56614e14ee8ae6c5044ad495c61456348
Bug-Url: https://bugzilla.redhat.com/1037663
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>