Libraries: upgrade Polymer 0.1.1 to 0.2.3

The current version of Polymer offered by JSBin is approximately [5 months old](https://github.com/cdnjs/cdnjs/tree/master/ajax/libs/polymer/0.1.1). This upgrades the CDNJS paths to point to the latest versions of Platform.js and Polymer.js (0.2.3).

:sunglasses: