Adds support for Django 1.9 while dropping support for unsupported versions.

[Dunedan]

This commit adds support for Django 1.9. As the template configuration options changed with Django 1.8 and versions earlier than 1.8 aren't supported anymore anyway by upstream, support for them is removed.

To have the Dockerfile still working, this commit also changes its logic so the dependencies get directly installed via the requirements.txt from PyPI, which will ensure that always matching versions of all dependencies are present (e.g. the Django version).

Building the Docker image will be broken until https://github.com/jforman/pybindxml/issues/15 is solved, as otherwise installing the obsolete lxml dependency fails.