Fixes tests broken by b89e5e0, part 2

Since URL patterns are cached, using `override_settings` to change `API_CURRENT_VERSION` for one module of tests does not also change the URL patterns dependent on that setting. This patch hardcodes the namespace into the reverses of that module (accomplishing the same thing as `override_settings`) and refactors `URLSerializerMixin` to be sensitive to the changes in b89e5e0 that affect URL reversal for different API versions.

r? @diox