Bump node-sass from 4.7.2 to 4.13.1

Bumps [node-sass](https://github.com/sass/node-sass) from 4.7.2 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@v4.7.2...v4.13.1)

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

Author: dependabot[bot]

package.json

@@ -26,7 +26,7 @@
     "file-loader": "^1.1.11",
     "html-webpack-plugin": "^3.1.0",
     "mini-css-extract-plugin": "^0.5.0",
-    "node-sass": "^4.7.2",
+    "node-sass": "^4.13.1",
     "postcss": "^6.0.21",
     "postcss-cssnext": "^3.1.0",
     "postcss-import": "^11.1.0",