Fix for Ruby 1.9.3-rc1

The executable was dying on me in Ruby 1.9.3-rc1, looks like an older method in the YAML module was being used. This was causing a `undefined method 'each_document' for Psych:Module (NoMethodError)`

I didn't find any tests for the executable, but running this at a bash prompt in the root of the project should verify it still works with multiple documents.

```
cat <<data | ./bin/mustache - ./examples/simple.mustache                                                                         
---
name: Awesome
value: 200
in_ca: true
taxed_value: 20
---
name: Awesome2
value: 400
in_ca: false
---
data
```