Update util.py
Update util.py

On CPython2 one should use `__builtin__` module, not a `__builtins__` dict.
kharandziuk/carbon,piotr1212/carbon,benburry/carbon,xadjmerripen/carbon,criteo-forks/carbon,graphite-project/carbon,JeanFred/carbon,benburry/carbon,obfuscurity/carbon,krux/carbon,graphite-server/carbon,krux/carbon,johnseekins/carbon,graphite-server/carbon,pu239ppy/carbon,lyft/carbon,pratX/carbon,deniszh/carbon,pratX/carbon,graphite-project/carbon,criteo-forks/carbon,mleinart/carbon,JeanFred/carbon,pu239ppy/carbon,deniszh/carbon,cbowman0/carbon,xadjmerripen/carbon,iain-buclaw-sociomantic/carbon,johnseekins/carbon,cbowman0/carbon,iain-buclaw-sociomantic/carbon,obfuscurity/carbon,mleinart/carbon,piotr1212/carbon,lyft/carbon,kharandziuk/carbon