Tell jshint about Handlebars global variable

This tells jshint about the Handlebars global variable.

Use case:

```javascript
// app/helpers/render-markdown.js

export default Ember.Handlebars.makeBoundHelper(function(markdown) {
  var showdown = new Showdown.converter();
  return new Handlebars.SafeString(showdown.makeHtml(markdown));
});
```