message: Update changelog for pre.13
diff: # Changelog
  -## Upcoming
  +## v0.18.0-pre.13 [03-08-2017]
  - When running `polymer build` and compiling JS to ES5, we will also rewrite script includes of `webcomponents-loader.js` to `webcomponents-es5-loader.js`.
  -## v0.18.0-pre.11 [03-07-2017]
  +## v0.18.0-pre.12 [03-07-2017]
  - Add PSK 3.0 (Polymer 2.0 Polymer Starter Kit) template to the init command.
  - Automatically include un-optimized `webcomponentsjs` polyfills in builds.
  