Bump node-sass from 4.12.0 to 4.13.1 (#275)

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] <[email protected]>

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

Author: dependabot[bot]

package-lock.json

@@ -22,7 +22,7 @@
     "babel-register": "^6.26.0",
     "css-loader": "^2.1.1",
     "mini-css-extract-plugin": "^0.4.5",
-    "node-sass": "^4.12.0",
+    "node-sass": "^4.13.1",
     "optimize-css-assets-webpack-plugin": "^5.0.1",
     "sass-loader": "^7.1.0",
     "uglifyjs-webpack-plugin": "^2.1.3",