Bump node-sass from 4.13.0 to 4.13.1 in /demo

Bumps [node-sass](https://github.com/sass/node-sass) from 4.13.0 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@v4.13.0...v4.13.1)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

demo/package-lock.json

@@ -32,7 +32,7 @@
     "@babel/preset-env": "^7.6.3",
     "babel-loader": "^8.0.6",
     "css-loader": "^3.2.0",
-    "node-sass": "^4.13.0",
+    "node-sass": "^4.13.1",
     "sass-loader": "^8.0.0",
     "vue-loader": "^15.7.1",
     "vue-style-loader": "^4.1.2",