opencensus-ext-django: upgrade pre-Django 1.10-style middleware

[JeremyVriens]

Background
Currently the opencensus-ext-django package has a version pin on django>=1.11, but it still uses the pre-Django 1.10-style middleware.
Since the current base class of the middleware is deprecated (assuming by the fact that it's loaded from the django.utils.deprecation package), the deprecated class can be removed at any point in time in the future.

Proposed solution
The middleware class should be refactored to the "new way of writing middlewares" as described in the Django documentation.

Acceptance criteria
No breaking changes. The refactor should be fully backwards compatible with the current middleware class.

[JeremyVriens]

I created this feature request/issue based on the findings after fixing #1154 and I will pick this one up myself as well.