Allow updates for core plugins

Core plugins (those with the `*.jpi`) are automagically managed by Jenkins by default. In order to update them manually (the method `puppet-jenkins` uses), they require a `pluginname.jpi.pinned` file to be in the plugins directory, which has the effect of putting them under manual control.

This PR causes the `.pinned` file to be created when core plugins are managed either at the latest version or at a specific version. It also pulls in the latest commits from upstream.