BUG: Fix price caching for tickers with '/' char
BUG: Fix price caching for tickers with '/' char

On Ubuntu (assume this is true for all posix) tickers containing a slash char ("CRD/A", "BRK/A", both valid tickers with yahoo api accessible timeseries) lead to a path error in loader.py line 286.

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