Add support for Sass 3.4, and disable sourcemaps

This uses the latest available version of gulp-ruby-sass, so that we can test the Sass 3.4 integration easily.

As soon as it enters 1.0 release, we can put this into master.

(note: this is not a PR to go into master, but a separate testing branch: "sass-34")
Master works well for now, but if 1.0 happens to fix bugs that we find, then sure.