Update to include "Implement Clone for histograms".

https://github.com/cms-sw/root/commit/0e440ee5f066ea63115eb843366bd23d5c40fe42.
Merge pull request #1127 from degano/root_53418_73XStable

Update to include "Implement Clone for histograms".
Merge remote branch 'origin/IB/CMSSW_7_3_X/stable' into IB/CMSSW_7_0_X/stable