Please use gopkg.in for importing mgo

[Ian Booth]

> The following import paths should be used instead:
> 
> gopkg.in/mgo.v2
> gopkg.in/mgo.v2/bson
> gopkg.in/mgo.v2/txn
> 
> This was perhaps not publicised well enough but the switch was made a
> couple of weeks ago.
>

FWIW, the switch was only done yesterday in order to pick up the new mgo driver
version needed to fix a mongo panic, and the changes were only done on Github
and not Launchpad, hence the import path change.