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.
Devise.use_salt_as_remember_token is removed
In Devise 2.0, the only acceptable value for this option was true, since we no longer maintained a remember_token.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).
Encryptable now is in a gem
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 format now returns a contentless header
Now the sign_out through JSON or XML returns a contentless response instead of a response with an empty body.