Fix "from urls import ..." -> "from readthedocs.urls import ..."

Related to #1496.

Author: gregmuellegger

readthedocs/core/subdomain_urls.py

@@ -1,7 +1,7 @@
 from django.conf.urls import url, patterns
 
 from readthedocs.projects.constants import LANGUAGES_REGEX
-from urls import urlpatterns as main_patterns
+from readthedocs.urls import urlpatterns as main_patterns
 
 handler500 = 'readthedocs.core.views.server_error'
 handler404 = 'readthedocs.core.views.server_error_404'