upgrade travis to use python 3.5, instead of python 3.4
upgrade travis to use python 3.5, instead of python 3.4

hughperkins/cuda-on-cl,hughperkins/cuda-on-cl,hughperkins/coriander,hughperkins/cuda-on-cl,hughperkins/coriander,hughperkins/coriander,hughperkins/coriander,hughperkins/cuda-on-cl