Update Juju 1.20 to work with latest mgo

This required to deal with a mgo bug (LP #1318366). The mgo repo
changed location after 1.20 so this update is more complex than it
otherwise would have been.

Although the change is mostly about dependencies and import paths,
some test changes were also required due to changes in mgo's
implementation.
Merge pull request #435 from mjs/1.20-mgo-fix

Update Juju 1.20 to work with latest mgo

This required to deal with a mgo bug (LP #1318366). The mgo repo changed location after 1.20 so this update is more complex than it otherwise would have been.

Although the change is mostly about dependencies and import paths, a test change was also required due to changes in mgo's internal behaviour.