We're supposing that you are currently using Devise 2.0, if not, before doing any of the tips below, upgrade your app to Devise 2.0
If you run your application with Devise 2.0 and have removed all the deprecation warnings, you're good to go with Devise 2.1.
In Devise 2.0, the only acceptable value for this option was true, since we no longer maintained a remember_token. Since it always had to be true, the option was removed all together (we had planned a two steps deprecation since 2.0 release).
If you're using encryptable on your models, you will now have to include
devise-encryptable on your gemfile.
sign_out through JSON or XML returns a contentless response instead of a response with an empty body.
views/devise/_links.html.erb has been moved back to
views/devise/shared/_links.html.erb so if you
updated customized views in 2.0 you will need to update the path.