Can't get Emoijone ckeditor plugin to appear on WSYISYG editor
You need to add annotation for ckeditor plugin (more info about annotations based plugins here). 

Youtube plugin is using this:

/**
 * Defines the "Youtube Button" plugin.
 *
 * @CKEditorPlugin(
 *   id = "youtube",
 *   label = @Translation("Youtube Plugin")
 * )
 */
class YoutubePlugin extends CKEditorPluginBase {


So you will need to add an annotation like this:

 /**
  * Defines the "Emojione" plugin.
  *
  * @CKEditorPlugin(
  *   id = "emojione",
  *   label = @Translation("Emojione")
  * )
  */
 class EmojionePlugin extends CKEditorPluginBase {


EDIT (thanks @kevin): clear caches after changes.