[FIXED JENKINS-16544] Use Shade plugin to avoid breaking Jenkins CLI

Second part of [JENKINS-16544](https://issues.jenkins-ci.org/browse/JENKINS-16544) fix, after  https://github.com/kohsuke/file-leak-detector/pull/6.

(Obviously requires `1.6-SNAPSHOT` to be updated to `1.6` when the first is accepted and released.)