[travis] force protobuf 3.0.0b2 for Python 3
[travis] force protobuf 3.0.0b2 for Python 3

This is temporary measure to avoid an apparent upstream issue with
protobuf 3.0.0b2.post1.

gnina/gnina,tackgeun/caffe,tackgeun/caffe,gnina/gnina,CZCV/s-dilation-caffe,CZCV/s-dilation-caffe,gnina/gnina,gnina/gnina,gnina/gnina,gnina/gnina,CZCV/s-dilation-caffe,tackgeun/caffe,CZCV/s-dilation-caffe,tackgeun/caffe