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"

yarn.lock

@@ -6132,10 +6132,10 @@ sort-keys@^1.0.0:
   dependencies:
     is-plain-obj "^1.0.0"
 
-sortablejs@^1.6.1:
-  version "1.7.0"
-  resolved "https://registry.yarnpkg.com/sortablejs/-/sortablejs-1.7.0.tgz#80a2b2370abd568e1cec8c271131ef30a904fa28"
-  integrity sha1-gKKyNwq9Vo4c7IwnETHvMKkE+ig=
+sortablejs@^1.8.1:
+  version "1.8.1"
+  resolved "https://registry.yarnpkg.com/sortablejs/-/sortablejs-1.8.1.tgz#d19b66cf10aab9521a565b79766f2f3291c18278"
+  integrity sha512-r0bKxZYGUFgv/6DQSWW8TVJBBtupUn6TPnDABd4rlqNq+u94ct5+barZmYauPAQNCKwy56C1kg9Y9msAN7UPag==
 
 source-list-map@^2.0.0:
   version "2.0.1"