travis: Make sure protobuf 2.6 is used
travis: Make sure protobuf 2.6 is used

The protobuf-compiler package provided in Travis environment is 2.4
right now which generates Python code incompatible with Python 3 (the
generated files refer to the unicode type, for example), protobuf
compiler 2.6 does the right thing.

smarkets/smk_python_sdk