Bump node-sass from 3.13.1 to 4.13.1 in /demo

Bumps [node-sass](https://github.com/sass/node-sass) from 3.13.1 to 4.13.1.
- [Release notes](https://github.com/sass/node-sass/releases)
- [Changelog](https://github.com/sass/node-sass/blob/master/CHANGELOG.md)
- [Commits](sass/node-sass@v3.13.1...v4.13.1)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]