Add a way to put some page on top of job bucket list

Fixing next job bucket list: use 'zadd' instead of 'sadd'
to ensure the elements order.
Adapt fixtures to changes in factory_boy 2.4.0

This commit also complies to currently deprecated changes [1]
introduced in the 2.4.0 release.

1: https://github.com/rbarrois/factory_boy/blob/master/docs/changelog.rst#240-2014-06-21