upgrade some JS deps to work with node 11

Fixes #62

* added travis config for latest node as well as 8.

* ran "yarn upgrade -P webpack-dev-server", which caused the upath
  dependency to be upgraded. I arrived at this by inspecting yarn.lock
  for the things depending on upack, along with some trial and error.
  ("yarn upgrade -P chokidar" was less successful.)

.travis.yml

@@ -33,6 +33,12 @@ matrix:
         - ci/install-ffmpeg.sh
       script:
         - ci/script-rust.sh
+    - language: node_js
+      node_js: "node"
+      script:
+        - yarn
+        - yarn build
+        - node_modules/eslint/bin/eslint.js ui-src
     - language: node_js
       node_js: "8"
       script: