Intentionally sacrifices some code coverage of the rebuild plugin so that
I don't need to find an alternate way to invoke rebuild.  The previously
tested web action is intentionally disallowed by SECURITY-3033 as
implemented in jenkinsci/rebuild-plugin#155

Silence compiler warnings

https://stats.jenkins.io/pluginversions/promoted-builds.html shows that
85% of the installations of the most recent release (delivered 3 months
ago) are already running Jenkins 2.440.3 or newer.  Users that are
upgrading to the most recent release of the plugin are also upgrading
to the most recent Jenkins releases.

Removes unnecessary declarations of mockito-core and hamcrest since
those are provided by the parent pom.

Removes unnecessary exclusions from other dependencies.

jenkinsci/rebuild-plugin#155 uses the "Rebuild"
button to rebuild the job.  Use the same technique here.