Add CMSSW_7_3_X IB for slc6_amd64_gcc491
Revert to use stable root in CMSSW_7_2_THREADED_X
Merge pull request #1018 from ktf/revert-root-threaded

Revert to use stable root in CMSSW_7_2_THREADED_X
Merge remote branch 'origin/IB/CMSSW_7_0_X/stable' into IB/CMSSW_7_0_X/stable