Add django 1.11 support

[wjdp]

Will close #83.

Issue is django removed the format_output method from forms.MultiWidget, see https://docs.djangoproject.com/en/1.11/releases/1.11/#changes-due-to-the-introduction-of-template-based-widget-rendering.

I've tried to keep support for both >1.11 and <=1.10 by adding a template_name and get_context to GeopositionWidget but leaving the existing format_output.

It's working on 1.11, have not checked on 1.10 or 1.8.

[philippbosch]

Thanks @wjdp for this!!
I'm merging this into a django-1.11 branch first in order to change the test settings and include 1.11 before merging into master.

[rcotrina94]

[...] change the test settings and include 1.11 before merging into master.

Done in #85 and #86 :)