So after Lucifer got out, I'm wondering why hasn't he bought Ruby back, since she is THE MAIN REASON for his freedom.
In the last episode of season 4 Ruby was killed using the demon-killing-knife, also known as "Ruby's knife". It is stated in the show that this knife has the ability to kill demons, not just exercise them to hell. Therefore, she can not be brought back.
Her entry on the Supernatural wiki also states that she is deceased.
Edit: In the latest seasons the concept of "The Empty" has been introduced into the mythology of supernatural. It is still unclear if demons killed by the demon knife or the colt are sent here.