Stop using removed `Configurator.with_context` method

Removed method is used here:

https://github.com/Pylons/pyramid_layout/blob/master/pyramid_layout/zcml.py#L40

See Pyramid 1.3a9 release notes:

http://docs.pylonsproject.org/projects/pyramid/en/latest/changes.html#id24

```
File "path/to/environment/lib/python3.3/site-packages/pyramid_layout/zcml.py", line 76, in layout
    config = Configurator.with_context(config_context)
zope.configuration.xmlconfig.ZopeXMLConfigurationError: File "path/to/project/commons/configure.zcml", line 13.4-14.62
    AttributeError: type object 'Configurator' has no attribute 'with_context'
```