Bump sortablejs from 1.14.0 to 1.15.0

Bumps [sortablejs](https://github.com/SortableJS/Sortable) from 1.14.0 to 1.15.0.
- [Release notes](https://github.com/SortableJS/Sortable/releases)
- [Commits](https://github.com/SortableJS/Sortable/compare/1.14.0...1.15.0)

---
updated-dependencies:
- dependency-name: sortablejs
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

package.json

@@ -16,7 +16,7 @@
     "opensans-webkit": "^1.1.0",
     "popper.js": "^1.16.1",
     "rails-erb-loader": "^5.5.2",
-    "sortablejs": "^1.14.0",
+    "sortablejs": "^1.15.0",
     "underscore": "^1.13.2",
     "vis": "^4.21.0",
     "webpack-merge": "^5.8.0"