Bump node-sass from 4.12.0 to 4.13.1 (#2)

Bumps [node-sass](https://github.com/sass/node-sass) from 4.12.0 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@v4.12.0...v4.13.1)

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