title: Can't log in to existing Clearance accounts after upgrading from Rails 2.3 to 4.1
body: <p>Working on upgrading an old Rails 2.3 application to use Rails 4. Everything is running just fine so far, except that after upgrading Clearance to the latest stable version I am no longer able to log in to existing user accounts. There's no error, other than incorrect email or password. I can also create new accounts and log in to them just fine, but the existing ones are unaccessible and I'd like to not have to reset hundreds of passwords for the users if at all possible.</p>
  
  <p>I know that Clearance started using BCrypt instead of SHA1 in the versions used after Rails 2.3, so I have set the <code>config.password_strategy = Clearance::PasswordStrategies::SHA1</code> in my <code>/config/initializers/clearance.rb</code> in order to have it use the same encryption that the accounts were created with. But this hasn't solved the problem.</p>
  
  <p>Any suggestions? I feel like some other sort of "secret" key was set somewhere in the old application that I need to bring over to the new application in order for the login decryption to work, but I don't know much about that kind of thing yet.</p>
  
  <p>Thanks!</p>