Adjust template to Polymer/platform having moved to Polymer/webcomponents #2

TBR: expediting this per request from the Polymer team for a GDG event tomorrow.

It turns out Polymer is in a bit of a flux right now: the switch to Polymer/webcomponents is incomplete; Polymer/webcomponents itself is not officially released yet, so Polymer/paper-elements still depend on Polymer/platform, but Polymer/polymer no longer includes /platform, so it needs to be imported explicitly in bower.json.

After the switch is complete, this PR should be undone to the way the code was right before it.