Fix for "undefined method `new' for Guard::Middleman:Module"

Imitating guard/guard-coffeescript@6326cfec to resolve this error from `guard`:

```
14:14:07 - ERROR - Invalid Guardfile, original error is:
> [#] undefined method `new' for Guard::Middleman:Module
14:14:07 - ERROR - No guards found in Guardfile, please add at least one.
```