[Fixed JENKINS-21155] Update Swarm plugin to use Jenkins 1.480.3

The older dependency on Jenkins 1.424 does not include the latest
versions of the Apache commons httpclient, so it fails on some
remote procedures with codec related messages.
Merge pull request #8 from MarkEWaite/JENKINS-21155-update-jenkins-dependency

[Fixed JENKINS-21155] Update Swarm plugin to use Jenkins 1.480.3