You should install the atom-k9 package (to run watch sketches from the editor).
You may also want the set-syntax package to make it easy to select the JRubyArt syntax mode.
Note: You can easily edit language-jruby-art.cson to create custom snippets see extract below:-
'.source.ruby.processing': Background: prefix: 'bkg' body: 'background ${1:int}'