Update dependency @ibm/plex to v6.4.1 (#20)

Author: gliptak

webapps/frontend/package-lock.json

@@ -20,7 +20,7 @@
   "dependencies": {
     "@bpmn-io/dmn-migrate": "0.5.0",
     "@bpmn-io/form-js": "1.8.7",
-    "@ibm/plex": "6.4.0",
+    "@ibm/plex": "6.4.1",
     "angular": "1.8.2",
     "angular-animate": "1.8.2",
     "angular-cookies": "1.8.2",