Simplify detection of cmssw environment.
Merge pull request #3175 from alja/from-CMSSW_6_2_7

6_2_X cmsShow -- simplify detection of cmssw environment.
Merge remote branch 'origin/CMSSW_6_2_X' into CMSSW_7_0_X
Merge pull request #138 from cms-sw/CMSSW_6_2_X_SLHC

Cmssw 6 2 x slhc