Bump node-sass from 4.11.0 to 4.13.1 in /slides

Bumps [node-sass](https://github.com/sass/node-sass) from 4.11.0 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@v4.11.0...v4.13.1)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]