Fix TypeError: __init__() missing 1 required positional argument: 'env' on Python 3.4.1

I got strange error on Python 3.4.1 (installed by pyenv on Max OS X 10.9) with pyramid_webassets:
```python
  File "./Quest/Python/pyramid_webassets/pyramid_webassets/__init__.py", line 290, in includeme
    assets_env = get_webassets_env_from_settings(config.registry.settings)
  File "./Quest/Python/pyramid_webassets/pyramid_webassets/__init__.py", line 222, in get_webassets_env_from_settings
    assets_env = Environment(asset_dir, asset_url, **kwargs)
  File "./Quest/Python/webassets/src/webassets/env.py", line 750, in __init__
    super(Environment, self).__init__(**more_config)
  File "./Quest/Python/webassets/src/webassets/env.py", line 707, in __init__
    self.config.setdefault('resolver', self.resolver_class())
TypeError: __init__() missing 1 required positional argument: 'env'
```
This patch just fix it. Still not sure what happening there.