Bump gulp-ruby-sass and stylish deps.

@sindresorhus I noticed that in https://github.com/sindresorhus/gulp-ruby-sass/releases/tag/v0.6.0, a number of breaking changes were introduced in gulp-ruby-sass from the version we're currently using. Specifically:

> gulp-ruby-sass now operates as a true gulp streaming plugin. Instead of reading the file system in order to compile each file from gulp.src() it uses only the files in the stream for compilation. You'll need to upgrade your gulp.src() globs to contain all the files Sass needs to compile your stylesheets (including Ruby environment files like the Gemfile and .ruby-version if used).

I wasn't quite sure whether to interpret this as a) you need to be more explicit with source input or b) you need to be more explicit about environment configuration. Do you have insight into whether there are significant updates we would need to make on our end to switch to this version?

Would love to get it in place sooner if we eventually need to make updates so we can take into account the work @gauntface is doing on SUIT support in branch.