Change ses collection

See https://bugs.launchpad.net/oq-engine/+bug/1375788
The tests are running here: https://ci.openquake.org/job/zdevel_oq-engine/722

Notice the companion PR https://github.com/gem/oq-nrmllib/pull/134
Let's do the change in a separate pull request: https://bugs.launchpad.net/oq-engine/+bug/1377938