Handle multi-series charms in 1.25

[Cheryl Jennings]

Juju 2.0 introduced a new feature where charms can declare multiple series in metadata.yaml. Unfortunately, this means that these charms break on juju < 2.0 with an error like:

WARNING failed to load charm at "/home/ubuntu/temp/trusty/ubuntu": metadata: series: expected string, got []interface {}([]interface {}{"precise", "trusty", "wily", "xenial"})
ERROR charm not found in "/home/ubuntu/temp": local:trusty/ubuntu

1.25 should be updated to allow for both a string series, and a list. If a list is present in metadata.yaml, the first item on the list will be selected as the series for the charm.

[Martin Packman (gz)]

Update for charm.v5:

https://github.com/juju/charm/pull/203

Dependency update for juju:

https://github.com/juju/juju/pull/4918

[Ryan Beisner (1chb1n)]

Perfect-o. :-)

Linking original bug for reference: https://bugs.launchpad.net/juju-core/+bug/1545686.