Pin Polymer version in templates to `latest`

...now that the GitHub access rate limit is no longer an issue, which was the reason for the original back-and-forth between master-latest-master (master doesn't go through version resolution, so didn't hit the limit). The rate limit was fixed in #3733.
Merge pull request #3758 from ussuri/pin-polymer-in-templates-2

Pin Polymer version in templates to `latest`