Set up IBs for CMSSW_7_4_ROOT6_X

in cmssw:

I created branch CMSSW_7_4_ROOT6_X from CMSSW_7_3_ROOT6_X

https://github.com/cms-sw/cmssw/compare/CMSSW_7_4_ROOT6_X...CMSSW_7_3_ROOT6_X

The automatic merges should merge everything from CMSSW_7_4_X into CMSSW_7_4_ROOT6_X

in cmsdist:

I created IB/CMSSW_7_4_X/root6 from IB/CMSSW_7_3_X/root6

And then I merged IB/CMSSW_7_4_X/stable into IB/CMSSW_7_4_X/root6

in this pull request: https://github.com/cms-sw/cmsdist/pull/1221