REST: Fix versioning

REST: Fix versioning

One of the few remaining warts in the API is versioning. There is some
basic versioning there, but it doesn't work properly as-is. Fix this by
correcting the index endpoint ('/') to use Django REST Framework's
'reverse' function [1], which handles versioning for us [2] and
switching from 'NamespaceVersioning' to 'URLPathVersioning', the latter
of which does the same thing but in a different way.

[1] http://www.django-rest-framework.org/api-guide/reverse/#reverse
[2] http://www.django-rest-framework.org/api-guide/versioning/#reversing-urls-for-versioned-apis
[3] http://www.django-rest-framework.org/api-guide/versioning/#urlpathversioning

Signed-off-by: Stephen Finucane <stephen@that.guru>