Fix multithreading issue in TFormula exposed by Vincenzo.

Move to cms-sw/root@67f4a75ab1.
Merge pull request #1015 from ktf/fix-tformula

Fix multithreading issue in TFormula exposed by Vincenzo.
Merge remote-tracking branch 'origin/IB/CMSSW_7_2_X/stable' into IB/CMSSW_7_0_X/stable