Setup Travis CI build system.
Add in hopefully correct dependencies.
Add back xetex.
Build script now terminates on error.
Merge branch 'f/terminate-build-script' into upstream/build-system
The "script" field no longer does cd nonsense.
Add texlive-math-extra as a dependency
Add the cd stuff back in.

it turns out it wasn't pointless. it was needed because the build script assumes
you are in its directory.
Add texlive-latex3 as a dep
Merge branch 'build-system' into develop
Merge branch 'build-system' into develop
Merge branch 'develop' of github.com:learnmath/lysa into develop
Merge branch 'develop' into f/peano