Bump swarm-client plugin

[sbesson]

See https://jenkins.io/security/advisory/2017-10-11/#swarm-plugin-client-bundled-vulnerable-version-of-the-commons-httpclient-library

The version of the Swarm plugin will need to be upgraded to 3.5 or above (currently 3.6). The client plugin should be upgraded in coordination which probably requires bumping https://github.com/openmicroscopy/devslave-c7-docker/blob/master/Dockerfile#L28 (or rebuilding the image with a different argument if possible).

[joshmoore]

#91 upgrades the swarm plugin, but likely we should just remove it. We are no longer using it in favor of nodes which autoconnect over a docker network.

[sbesson]

Closing as addressed by 0.7.0, we can capture the proposed cleanup in a separate issue.