Update guide.html to use "language: rust" in .travis.yml

travis-ci now supports "language: rust", which eliminates the need to call rustup.sh in .travis.yml.  See http://www.reddit.com/r/rust/comments/2ecocp/travis_ci_supports_rust/ for details.
It does now: https://github.com/travis-ci/travis-build/pull/286 though this change may not be deployed yet.