Prepare for PyMongo 2.9

[MRigal]

Hi guys,

PyMongo 2.9 is approaching, rc0 is out. It provides also the following (which ideally should have come BEFORE Pymongo3 :-p ) http://api.mongodb.org/python/2.9rc0/migrate-to-pymongo3.html

We will need some last adaptations, maybe write some documentation and add 2.9 to Travis

[wojcikstefan]

We already support PyMongo v3.0