all: use gopkg.in/juju/charm.v2

Since we already depend on gopkg.in, this should not
be blocked for the same reason as previously.
testing/factory: remove outdated reference to old charm package path
Merge pull request #253 from rogpeppe/012-charm-v2

all: use gopkg.in/juju/charm.v2

We use the new name for the charm package, and update
all places to use the new API.