Update django-registration-redux 1.4 -> 1.6

The Travis test matrix for version 1.6 includes Django 1.11.

There are breaking changes listed in the django-registration-redux
CHANGELOG.
- Remove Django 1.7 support. No problem.
- Change the signature of the `activate_user` function. We do not call
this function from our code.

requirements.txt

@@ -15,7 +15,7 @@ django-hstore==1.4.2
 django-js-reverse==0.7.3
 django-queryset-csv==1.0.0               # https://github.com/azavea/django-queryset-csv/commits/master
 django-redis==4.8.0
-django-registration-redux==1.4
+django-registration-redux==1.6
 django-statsd-mozilla==0.3.16
 django-storages==1.5.1
 django-threadedcomments==1.1