CI: setup 32-build linux build on travis
so if anyone is interested: https://github.com/matthew-brett/multibuild/tree/7abbd01f230be6067842f0510a98b06a6320ddf9

hosts the builder for the wheels, it does a 32-build on linux via a docker image. So we could just directly pull that same image and build (of master).

cc @matthew-brett 

xref #13584 
