Start by setting up combine in CMSSW 8, and then also set up combineHarvester:
source /cvmfs/cms.cern.ch/cmsset_default.sh
REL=$(lsb_release -r | awk '{print $2}')
if [[ $REL == 6* ]]; then
export SCRAM_ARCH=slc6_amd64_gcc530
elif [[ $REL == 7 ]]; then
export SCRAM_ARCH=slc7_amd64_gcc530
fi
cmsrel CMSSW_8_1_0
cd CMSSW_8_1_0/src
cmsenv
git clone https://github.com/cms-analysis/HiggsAnalysis-CombinedLimit.git HiggsAnalysis/CombinedLimit
cd HiggsAnalysis/CombinedLimit
git fetch origin
git checkout v7.0.13
scramv1 b clean; scramv1 b -j4
cd $CMSSW_BASE/src
git clone https://github.com/cms-analysis/CombineHarvester.git CombineHarvester
scram b -j4