Revert "Use the coq-git ppa on travis"

This reverts commit d17f47f34bc4352d9fa83fb97fa138c99b8d9fb8.

We probably don't actually want the pull requests to show red `x`s
whenever the most recent Coq trunk happens to break some random file in
HoTT.
Merge pull request #436 from JasonGross/revert-travis-ppa

Revert "Use the coq-git ppa on travis"

In case anyone's interested, in this case, the bug is https://coq.inria.fr/bugs/show_bug.cgi?id=3545.