Test both g++ and clang++ on Travis.
Add automake-1.12 tests on Travis.

This requires installing a PPA to get it, but it's worth it to be able
to use the TAP driver with automake. Hopefully, Travis will upgrade to
the next LTS and get a newer automake within a year, anyway.

Also, I didn't bother testing automake-1.12+clang, since that's
basically the same as automake-1.11+clang.