Bump underscore from 1.13.3 to 1.13.4

Bumps [underscore](https://github.com/jashkenas/underscore) from 1.13.3 to 1.13.4.
- [Release notes](https://github.com/jashkenas/underscore/releases)
- [Commits](https://github.com/jashkenas/underscore/compare/1.13.3...1.13.4)

---
updated-dependencies:
- dependency-name: underscore
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

package.json

@@ -17,7 +17,7 @@
     "popper.js": "^1.16.1",
     "rails-erb-loader": "^5.5.2",
     "sortablejs": "^1.15.0",
-    "underscore": "^1.13.3",
+    "underscore": "^1.13.4",
     "vis": "^4.21.0",
     "webpack-merge": "^5.8.0"
   },