Replace gulp-ruby-sass with gulp-sass...

...thereby removing our Ruby dependency.

[gulp-sass][1] depends on [node-sass][3], which is a Node.js binding to [libsass][2], which is a C implementation of the SASS compiler. It's faster and does not require Ruby.

[1]: https://www.npmjs.org/package/gulp-sass
[2]: http://libsass.org/
[3]: https://www.npmjs.org/package/node-sass