Bump juju/charm dependency to purge old goyaml import
Merge pull request #791 from bz2/1.20_dep_charm_yaml

Bump juju/charm dependency to purge old goyaml import

Building juju was still pulling in launchpad.net/goyaml - as well as the newer gopkg.in/yaml.v1 location.

Fixed in the 1.20 branch of charm by <https://github.com/juju/charm/pull/48>, this change just bumps the dependency to require that.