Chapter 9, recipe 4 is skipped on Travis Linux

[robertodr]

Chapter 9, recipe 4 is now skipped on Travis Linux because the version of Boost Python installed via APT links against system Python (2.7) rather than Python 3.5, which is the version we need.

Possible Solution

Compile Boost components we need (filesystem, unit_test_framework, python) on our own in the set up phase. We can cache the artifacts as we do with much of the other infrastructure.