Adds Travis CI build status to README.
Adds Travis CI build status to README.

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