Update node-sass/gulp-sass for compatibility with Node v12

[wincent]

Once node-sass is updated for compatibility with Node v12, we'll be able to update. Until then, we will need to remain on Node v11.

Upstream issue: sass/node-sass#2632

Note that we don't depend on node-sass directly, but rather get it via gulp-sass, so it's actually both that will need to be updated before we can move.