Fix engine_initialized? to remove Tilt warning message

The `HandlebarsTemplate.engine_initialized?` method was always returning false because it was looking for a method named Barber rather than the `Barber` module. This caused Tilt to always output these warning messages:
```
WARN: tilt autoloading 'json' in a non thread-safe way; explicit require 'json' suggested.
WARN: tilt autoloading 'barber' in a non thread-safe way; explicit require 'barber' suggested.
```