chore(gulp): include `node_modules` in sass paths

[rekomat]

Just a proposal: Include node_modules in Sass include_path to make it more comfortable to import Sass partials from node_modules and prevent errors if the imported Sass files refer to other partials (like @material does).

@import '@material/select/mdc-select';
// instead of 
@import 'node_modules/@material/select/mdc-select';
// which throws an error as the partial refers to `@material/…`