all: use charm.v3

Change to use charm.Reference throughout.
update dependencies.tsv
update dependencies.tsv for new charm.v3 version
Merge pull request #49 from rogpeppe/020-use-charm.v3

all: use charm.v3

Change to use charm.Reference throughout.

dependencies.tsv not changed because charm.v3 has not yet landed, so
we don't have the commit ref.
Merge branch 'v4' into 021-http-status

Conflicts:
	internal/router/router.go
	internal/router/router_test.go
	internal/v4/api.go
rename charm.URL to charm.Reference after merge