Use a debian repository for HoTT/coq

Edward Yang (@ezyang) helpfully packaged HoTT/coq a while back.  The
category theory is getting slow enough that the build that generates the
html periodically fails, and I have to kick travis.  This should buy us
another 5-10 minutes.

However, @mattam82 tells me that he's hoping to get the bugs in the polyproj branch ironed out and universe polymorphism + primitive projections + local/private inductive types merged into trunk in the very near future, so we might not be using this version for long.  Hopefully by then @ezyang (or @ezyang and I) will have gotten [a ppa for Coq trunk](https://code.launchpad.net/~ezyang/+recipe/coq-daily) working, and so we'll be able to smoothly switch to that.