juju/charm master has changed

[Martin Packman]

Earlier I changed the revision that the master branch of the
github.com/juju/charm project refers to. In practical terms, I hope
this should not matter to anyone, as the v<N> branches are the
actively developed ones under the new versioning scheme. If there is
any issue, please let me know.

The reason for doing it, despite master basically being an abandoned
branch, is 'master' still has a special status. When fetched using go
get not using the redirector (as is the case for the 1.20 branch)
master is the initial state. As that still imported
launchpad.net/goyaml the juju tarball build process was pulling in
that as well. So, to avoid that issue I have pushed the current tip of
1.20 over master.

Martin