Bump webpack-merge from 4.1.2 to 4.1.3 in /js

Bumps [webpack-merge](https://github.com/survivejs/webpack-merge) from 4.1.2 to 4.1.3.
- [Release notes](https://github.com/survivejs/webpack-merge/releases)
- [Changelog](https://github.com/survivejs/webpack-merge/blob/master/CHANGELOG.md)
- [Commits](https://github.com/survivejs/webpack-merge/compare/v4.1.2...v4.1.3)

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

js/package.json

@@ -26,7 +26,7 @@
     "node-sass": "^4.5.3",
     "webpack": "^4.12.0",
     "webpack-cli": "^3.0.8",
-    "webpack-merge": "^4.1.2",
+    "webpack-merge": "^4.1.3",
     "karma": "^2.0.3"
   },
   "scripts": {