Correct URL regex for reviews

Had fallen out of sync with the core product URLs as it still used a
hyphen instead of an underscore.

oscar/apps/catalogue/app.py

     def get_urls(self):
         urlpatterns = super(ReviewsApplication, self).get_urls()
         urlpatterns += patterns('',
-            url(r'^(?P<product_slug>[\w-]*)-(?P<product_pk>\d+)/reviews/',
+            url(r'^(?P<product_slug>[\w-]*)_(?P<product_pk>\d+)/reviews/',
                 include(self.reviews_app.urls)),
         )
         return self.post_process_urls(urlpatterns)

oscar/apps/catalogue/reviews/app.py

-from oscar.core.application import Application
 from django.conf.urls import patterns, url
+
+from oscar.core.application import Application
 from oscar.apps.catalogue.reviews import views