Fix usage of render_to_string
Fix usage of render_to_string

This function signature changed in Django 1.10:

    https://docs.djangoproject.com/en/1.10/releases/1.10/#features-removed-in-1-10

Fortunately, it is possible to have it work both with our current Django
1.9 and the future 1.10 quite easily, which this commit does.

ideascube/ideascube,ideascube/ideascube,ideascube/ideascube,ideascube/ideascube