Update stylelint requirement to 9.1

Updates the requirements on [stylelint](https://github.com/stylelint/stylelint) to permit the latest version.
- [Release notes](https://github.com/stylelint/stylelint/releases)
- [Changelog](https://github.com/stylelint/stylelint/blob/master/CHANGELOG.md)
- [Commits](https://github.com/stylelint/stylelint/commits/9.1.3)

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

package.json

@@ -9,7 +9,7 @@
     "react": "16.1.0"
   },
   "dependencies": {
-    "stylelint": "8.0",
+    "stylelint": "9.1",
     "webpack": "^2.5.1"
   }
 }