DOC: Add libfreetype6-dev to debian reqs.
DOC: Add libfreetype6-dev to debian reqs.

alphaBenj/zipline,umuzungu/zipline,nborggren/zipline,humdings/zipline,florentchandelier/zipline,grundgruen/zipline,quantopian/zipline,magne-max/zipline-ja,quantopian/zipline,alphaBenj/zipline,magne-max/zipline-ja,wilsonkichoi/zipline,nborggren/zipline,grundgruen/zipline,Scapogo/zipline,florentchandelier/zipline,bartosh/zipline,humdings/zipline,enigmampc/catalyst,umuzungu/zipline,wilsonkichoi/zipline,Scapogo/zipline,bartosh/zipline,enigmampc/catalyst