Asciidoctor upgrade

Thanks for the upgrade and cleanup! About the other issue, I'm not sure whether it's a big problem to need to specify the paths to the gems explicitly, unless there's a reason I don't know about, it's not going to be the main focus of new features I'm planning for the plugin, e.g., I want to add code completion and other editor features and expose as much as possible of relevant features from gems to be included in different ways, via selection from dropboxes in the toolbar, etc.