Bump sortablejs from 1.7.0 to 1.8.1

Bumps [sortablejs](https://github.com/SortableJS/Sortable) from 1.7.0 to 1.8.1.
- [Release notes](https://github.com/SortableJS/Sortable/releases)
- [Commits](https://github.com/SortableJS/Sortable/compare/1.7.0...1.8.1)

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

package.json

@@ -31,7 +31,7 @@
     "messenger": "git+https://github.com/HubSpot/messenger.git",
     "moment": "^2.17.1",
     "promise": "^7.3.1",
-    "sortablejs": "^1.6.1",
+    "sortablejs": "^1.8.1",
     "sweetalert2": "^6.8.0",
     "vue": "^2.5.17",
     "vue-template-compiler": "^2.5.16"