Update Bower configuration.

* Use Handlebars `~>1.1.2` (will allow everything `< 1.2`).
* Replace `ember-data-shim` with `ember-data`. @fivetanley had the Bower
  registry fixed so that `ember-data-shim` and `ember-data` both point
  to https://github.com/components/ember-data. (FYI - Sometime in the future we
  will be removing `ember-data-shim`.)