To take into account the change in TMath::Erf generate a new reference file

stressRooFit_ref_2.root used by versions >=519 and restore the old reference file
for previous versions.


git-svn-id: http://root.cern.ch/svn/root/trunk@22649 27541ba8-7e3a-0410-8455-c3a389f83636