Verify that production and doc builds still work in CI

When I did the last `npm audit fix`, it completely escaped my
attention that this might break the scripts that we only run prior to
a release. This won't happen to me again!

.travis.yml

@@ -22,6 +22,9 @@ before_script:
 script:
   - "[ $EXTRA = false ] || npm run lint"
   - npm run prepare-tests
+  - "[ $EXTRA = false ] || npm run build-umd"
+  - "[ $EXTRA = false ] || npm run build-esm"
+  - "[ $EXTRA = false ] || npm run doc"
   - "[ $EXTRA = true ] || npm run test-node"
   - "[ $EXTRA = false ] || npm run coveralls"
   - "[ $EXTRA = false ] || npm run test-browser"