Update guide.md 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.

This is the same change as #425, but on the master branch in guide.md, not guide.html.