Trying to fix Readthedocs build

requirements/default.txt

@@ -1,3 +1,3 @@
 pytz>dev
-billiard>=3.3.0.17,<3.4
-kombu>=3.0.15,<4.0
+billiard>dev
+kombu>dev

requirements/docs.txt

@@ -1,5 +1,5 @@
-billiard
 Sphinx
 SQLAlchemy
+https://github.com/celery/billiard/zipball/master
 https://github.com/celery/py-amqp/zipball/master
 https://github.com/celery/kombu/zipball/master