Adding cffi as a requirement for python build.
Adding cffi as a requirement for python build.

kotfic/girder,msmolens/girder,jbeezley/girder,Kitware/girder,sutartmelson/girder,chrismattmann/girder,salamb/girder,salamb/girder,sutartmelson/girder,sutartmelson/girder,Xarthisius/girder,jcfr/girder,data-exp-lab/girder,jcfr/girder,manthey/girder,essamjoubori/girder,manthey/girder,Kitware/girder,adsorensen/girder,girder/girder,msmolens/girder,sutartmelson/girder,data-exp-lab/girder,opadron/girder,chrismattmann/girder,RafaelPalomar/girder,chrismattmann/girder,adsorensen/girder,chrismattmann/girder,data-exp-lab/girder,msmolens/girder,Xarthisius/girder,Xarthisius/girder,girder/girder,adsorensen/girder,kotfic/girder,adsorensen/girder,msmolens/girder,jbeezley/girder,opadron/girder,essamjoubori/girder,essamjoubori/girder,jcfr/girder,adsorensen/girder,kotfic/girder,girder/girder,kotfic/girder,jbeezley/girder,RafaelPalomar/girder,opadron/girder,manthey/girder,sutartmelson/girder,salamb/girder,Xarthisius/girder,RafaelPalomar/girder,Kitware/girder,salamb/girder,jcfr/girder,RafaelPalomar/girder,opadron/girder,kotfic/girder,essamjoubori/girder,jcfr/girder,Xarthisius/girder,salamb/girder,girder/girder,chrismattmann/girder,opadron/girder,data-exp-lab/girder,manthey/girder,Kitware/girder,essamjoubori/girder,RafaelPalomar/girder,jbeezley/girder,data-exp-lab/girder,msmolens/girder