Revert "Use Cython for convex hull."

This reverts commit 6324d48bd0ec7171aa5efe4c5baed6ca7ffe88dd.

Conflicts:
	.travis.yml
Revert "Try installing cython with pip"

This reverts commit fd493471bc12b8feded3d7b7efa8355f337783fc.
Revert "Install Cython on Heroku"

This reverts commit 2a192bc469a4311d0fc0b8dd64fc84f6da5499a0.