Bump node-sass from 3.3.3 to 4.14.1 in /reveal.js

Bumps [node-sass](https://github.com/sass/node-sass) from 3.3.3 to 4.14.1.
- [Release notes](https://github.com/sass/node-sass/releases)
- [Changelog](https://github.com/sass/node-sass/blob/master/CHANGELOG.md)
- [Commits](https://github.com/sass/node-sass/compare/3.3.3...v4.14.1)

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

reveal.js/package.json

@@ -39,7 +39,7 @@
     "grunt-autoprefixer": "~3.0.3",
     "grunt-zip": "~0.17.1",
     "grunt": "~0.4.5",
-    "node-sass": "~3.3.3"
+    "node-sass": "~4.14.1"
   },
   "license": "MIT"
 }