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 the version/tag resolution, so it didn't hit the limit, which was used as a quick workaround). The rate limit issue was fixed in #3733 by switching to `git ls-remote` instead of using the GitHub API.

TBR: @gaurave