Bump node-sass from 4.12.0 to 4.13.1 (#33)

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)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

package.json

@@ -46,7 +46,7 @@
         "jsdom": "9.4.5",
         "load-grunt-config": "0.19.2",
         "mini-css-extract-plugin": "0.8.0",
-        "node-sass": "4.12.0",
+        "node-sass": "4.13.1",
         "react": "15.1.0",
         "react-dom": "15.1.0",
         "sass-loader": "7.3.1",