See https://travis-ci.org/MacPython/matplotlib-wheels for details on how this all works.
## Notes
Differences from the standard ``multibuild`` instructions are noted in this section.*
Stan version 2.19 and higher requires C++14. In env_vars.sh we export MACOSX_DEPLOYMENT_TARGET=10.9
so that clang will use libc++ for the C++ standard library.
We also use a custom xenial image because to even test pystan we need a version of gcc more recent than the one that ships with trusty.