Revert "Update to gopkg.in/mgo.v2 repository"
Merge pull request #38 from juju/revert-33-update-mgo-gopkgin-v4

Revert "Update to gopkg.in/mgo.v2 repository"

Reverts juju/charmstore#33
Merge branch 'v4' into store-charm.
Reverted to original dependencies.