Use the coq-git ppa on travis

Now that ezyang's ppa works on precise, we can use it rather than
building Coq ourselves; this is about 10 minutes faster on each build.
We keep one test running Coq, to make sure that the pinned version of
Coq can always build the relevant version of the library.
Merge pull request #431 from JasonGross/travis-ppa

Great! Thanks @ezyang!
I am assuming we now have a ppa for Coq trunk.

Use the coq-git ppa on travis