I went into the gem's directory to add a plugins folder
app/assets/javascript/ckeditor/plugins.... I then added preview.js to
it. Afterwards, I added //= require ckeditor/plugins/preview.js to
ckeditor\application.js. Lastly, I added config.extraPlugins =
'preview'; to config.js. I still don't see the preview button or preview
pane for the gem.