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.

Also install libcoq-ocaml-dev
Add the -no-native-compiler argument to coqtop

It's required for a Coq built without that argument to not complain
about various things.
Merge pull request #358 from JasonGross/use-ppa

Use a debian repository for HoTT/coq