update the paper grind script

Update the grind script in the paper package, to delete un-needed files after an update to the latest polymer components (https://github.com/dart-lang/chromedeveditor/pull/3656).

@ussuri 
...actually anything starting with a `.`. Also Makefile, component.json, package.json (or even just `*.json`). I'm going through cde-polymer-designer.dart, that's why :)