Upgrade vendored handlebars.js to 1.0.0-rc.3

This fixes errors that were being thrown in the test suite, provided that you add `?handlebars=1.0.0-rc.3` to the URL.

I've also [submitted a pull request to ember-dev](https://github.com/emberjs/ember-dev/pull/3) to update the default handlebars version to 1.0.0-rc.3, which will remove the need for the extra query param.