Handle a None decorator in pyramid views

This is needed with pyramid 1.5a3. Without this change I get the following error on app startup:

```
  File "/Users/wichert/Library/buildout/eggs/pyramid-1.5a3-py2.7.egg/pyramid/config/views.py", line 1636, in add_forbidden_view
    return self.add_view(**settings)
  File "/Users/wichert/Library/buildout/eggs/appenlight_client-0.6-py2.7.egg/appenlight_client/ext/pyramid_tween.py", line 89, in wrapper
    kwargs['decorator'].append(wrap_pyramid_view_name)
AttributeError: 'NoneType' object has no attribute 'append'
```