tasks: add npm ls to preversion script

- npm ls exits with code 1 (i.e. errors out) when a unmet
  dependencies are present in node_modules/
- use this to as a sanity check before versioning and bundling

Author: etpinard

package.json

@@ -44,7 +44,7 @@
     "start-image_viewer": "node devtools/image_viewer/server.js",
     "start": "npm run start-test_dashboard",
     "baseline": "node tasks/baseline.js",
-    "preversion": "npm-link-check && npm dedupe",
+    "preversion": "npm-link-check && && npm dedupe && npm ls",
     "version": "npm run build && git add -A dist src build",
     "postversion": "git push && git push --tags"
   },