BLD: Use Python 3 on Travis.
BLD: Use Python 3 on Travis.

To help prevent regressions when a changes are developed using
Python 2.7, also test Python 3 during continuous integration.

sketchytechky/zipline,dkushner/zipline,YuepengGuo/zipline,Scapogo/zipline,florentchandelier/zipline,otmaneJai/Zipline,bartosh/zipline,dmitriz/zipline,ronalcc/zipline,wilsonkichoi/zipline,bartosh/zipline,aajtodd/zipline,gwulfs/zipline,jimgoo/zipline-fork,gwulfs/zipline,euri10/zipline,jordancheah/zipline,CDSFinance/zipline,zhoulingjun/zipline,DVegaCapital/zipline,enigmampc/catalyst,StratsOn/zipline,joequant/zipline,StratsOn/zipline,grundgruen/zipline,alphaBenj/zipline,enigmampc/catalyst,semio/zipline,chrjxj/zipline,michaeljohnbennett/zipline,wubr2000/zipline,dhruvparamhans/zipline,umuzungu/zipline,euri10/zipline,florentchandelier/zipline,MonoCloud/zipline,ChinaQuants/zipline,zhoulingjun/zipline,jordancheah/zipline,keir-rex/zipline,morrisonwudi/zipline,AlirezaShahabi/zipline,quantopian/zipline,semio/zipline,nborggren/zipline,iamkingmaker/zipline,stkubr/zipline,wubr2000/zipline,stkubr/zipline,cmorgan/zipline,ronalcc/zipline,humdings/zipline,dhruvparamhans/zipline,MonoCloud/zipline,keir-rex/zipline,dkushner/zipline,erikness/AlephOne,umuzungu/zipline,chrjxj/zipline,michaeljohnbennett/zipline,quantopian/zipline,grundgruen/zipline,YuepengGuo/zipline,magne-max/zipline-ja,kmather73/zipline,DVegaCapital/zipline,morrisonwudi/zipline,humdings/zipline,kmather73/zipline,sketchytechky/zipline,jimgoo/zipline-fork,iamkingmaker/zipline,erikness/AlephOne,mattcaldwell/zipline,nborggren/zipline,mattcaldwell/zipline,AlirezaShahabi/zipline,Scapogo/zipline,dmitriz/zipline,alphaBenj/zipline,cmorgan/zipline,aajtodd/zipline,magne-max/zipline-ja,CDSFinance/zipline,wilsonkichoi/zipline,joequant/zipline,otmaneJai/Zipline,ChinaQuants/zipline