Ignore sphinx 1.3b1

Sphinx 1.3b1 was released on oct 9th and seems to be causing errors
http://logs.openstack.org/16/120316/6/gate/gate-oslotest-docs/2d65dbe/console.html

Closes-Bug: #1379998

Change-Id: I1a52b4be02293e6c7ea5e54bafd7054fe9d0270d