Support for missing URL names in Django 1.8 and 1.9

[florisdenhengst]

URL names are optional in Django, but the silk.model_factory.construct_request_model method depends on the presence url_name in the resolved url.
This attribute is only set when a url_name is specified in the pattern for the requested url in Django >= 1.8.
If it is not set, the following error appears:

Traceback (most recent call last):
  File "[/path/to/django]/core/handlers/base.py", line 123, in get_response
    response = middleware_method(request)
  File "[/path/to/silk]/middleware.py", line 95, in process_request
    request_model = RequestModelFactory(request).construct_request_model()
  File "[/path/to/silk]/model_factory.py", line 166, in construct_request_model
    view_name = namespace + ':' + view_name
TypeError: Can't convert 'NoneType' object to str implicitly

The url_name is set by Django in versions 1.6 and 1.7 but not in 1.8 and 1.9.
In these latter two versions, a view_name is set instead.

It would be possible to use this view_name in construct_request_model whenever present (for Django>=1.8) or determine it as is currently done (for 1.6 >= Django > 1.8)