Upgrade Handlebars.js to v1.0.12

No test changes necessary, everything passes.

* [Handlebars v1.0.12 docs](https://github.com/wycats/handlebars.js/blob/master/release-notes.md#v1012--100---may-31-2013)
* [Handlebars v1.0.12 tag](https://github.com/wycats/handlebars.js/commits/v1.0.12)