Add gulp build to hint (#2629)

Author: piotr-cz

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 **Prior to filing a PR, please:**
 - [open an issue](https://github.com/GoogleChrome/workbox/issues/new) to discuss your proposed change.
-- ensure that `gulp lint test` passes locally.
+- ensure that `gulp build && gulp lint test` passes locally.
 
 R: @jeffposnick @philipwalton