Speed up Travis platform setup

[ian-ross]

There are a couple of Python packages that we use that don't have binary distributions (in particular, the SASS preprocessor we use). These currently have to be installed from source every time a Travis build starts, which is very slow. We should set up a "wheelhouse" to hold binary builds of packages to speed up Travis (and tox) setup. (Suitable tools to do this might be https://github.com/level12/wheelhouse or https://github.com/WhoopInc/mkwheelhouse)