Without this, using `configurations.setup()` after Django has been setup
already re-triggers the logging configuration, which then causes e.g.
pytest's caplog to not work anymore.
* Drop obsolete versions of Django from Travis tests
Django 1.8 reached EOL in April 2018.
Django 1.10 reached EOL in December 2017.
* Add support for testing Django 2.0 and 2.1 on Python 3.7
* Drop obsolete test (requires Django < 1.10)
Hopefully this saves time for new users of django-configuration (like myself), who "just needed" to lazily evaluate a string inside a dictionary.
This doubles as an example for `RAVEN_CONFIG` which was the whole reason I was here in the first place... The actual problem I faced was that a setting remains of type `values.Value` when nested inside a dictionary. Which results in a weird issues like this:
```
2018-02-24T20:59:26.125208+00:00 app[web.1]: Traceback (most recent call last):
2018-02-24T20:59:26.125364+00:00 app[web.1]: self.client.http_context(self.get_http_context(environ))
2018-02-24T20:59:26.125215+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/raven/middleware.py", line 98, in __call__
2018-02-24T20:59:26.125368+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/raven/contrib/django/models.py", line 54, in <lambda>
2018-02-24T20:59:26.125482+00:00 app[web.1]: __getattr__ = lambda x, o: getattr(get_client(), o)
2018-02-24T20:59:26.125486+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/raven/contrib/django/models.py", line 134, in get_client
2018-02-24T20:59:26.125613+00:00 app[web.1]: instance = Client(**options)
2018-02-24T20:59:26.125618+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/raven/contrib/django/client.py", line 147, in __init__
2018-02-24T20:59:26.125769+00:00 app[web.1]: Client.__init__(self, *args, **kwargs)
2018-02-24T20:59:26.125771+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/raven/base.py", line 171, in __init__
2018-02-24T20:59:26.125927+00:00 app[web.1]: self.set_dsn(dsn, transport)
2018-02-24T20:59:26.125929+00:00 app[web.1]: File "/app/.heroku/python/lib/python3.6/site-packages/raven/base.py", line 251, in set_dsn
2018-02-24T20:59:26.126063+00:00 app[web.1]: if dsn not in self._transport_cache:
2018-02-24T20:59:26.126076+00:00 app[web.1]: TypeError: unhashable type: 'Value'
```
