Enable gulp generate-typescript on Travis

This commit is expected to fail, as the library
code has been modified, but the task was
explicitly not run and files are not added.

.travis.yml

@@ -12,6 +12,7 @@ before_script:
 - npm install -g bower gulp-cli@1
 - bower install
 - gulp lint-eslint
+- gulp generate-typescript
 - git diff --exit-code
 script:
 - xvfb-run wct -l chrome

lib/utils/async.html

@@ -64,7 +64,7 @@
        * delay.
        *
        * @memberof Polymer.Async.timeOut
-       * @param {number=} delay Time to wait before calling callbacks in ms
+       * @param {number} delay Time to wait before calling callbacks in ms
        * @return {!AsyncInterface} An async timeout interface
        */
       after(delay) {