Use gopkg.in/mgo.v2 for mgo imports

[byrnedo]

Hi,

Great project :)

I was trying to integrate with some code of mine and I got
cannot use mongo.Conn() (type *"gopkg.in/mgo.v2".Session) as type "labix.org/v2/mgo".
I had trouble in the past and apparently it is suggested to use gopkg.in/mgo.v2 instead of labix.org/v2/mgo.

See http://stackoverflow.com/questions/31244438/mgo-newobjectid-corrupt-on-insert for the comment from mgo's maintainer.