Update handlebars

The version of handlebars used by Meteor is over a year old.
Many bugs have been fixed, new features added, etc.
The AST has changed slightly a year ago, so this requires (minimal) changes to the code.
The first commit is probably not what you want as it uses the stock handlebars. If you update your fork (should I create a PR there??) I'll modify it.
The second commit makes the small change needed to use the updated AST.