Dynamically select XZ BCJ filter

[JonathonReinhart]

Closes #53

[JonathonReinhart]

This is failing because SCons raised an exception:

ImportError: No module named backports:

pip install -r requirements.txt definitely ran and installed backports.lzma. However, I'm guessing it did so in a Python virtualenv, and not in the system's site packages, which scons would be using.

https://docs.travis-ci.com/user/languages/python/#Travis-CI-Uses-Isolated-virtualenvs

Even though we're in a virtual env, we may be able to forcefully use the system's pip (and thus install into site-packages) via:

$ env PATH=$_OLD_VIRTUAL_PATH which pip
/usr/bin/pip

[JonathonReinhart]

This hasn't been tested on any other architectures, but should be at least really close.