adding python 3 back into travis build
adding python 3 back into travis build

joezuntz/emcee,dfm/emcee,dfm/emcee,drphilmarshall/emcee,johnbachman/emcee,ipashchenko/emcee-x,pkgw/emcee,johnbachman/emcee,jellis18/emcee3,dfm/emcee3,ipashchenko/emcee,willvousden/emcee,farr/emcee,elkingtonmcb/emcee,ga7g08/emcee,linan7788626/emcee,willvousden/emcee