Fix migration 20101026184950 when running after rollback

When the `up` method of migration `20101026184950` runs after the rollback,
the column `unlock_token` already exists as it's added by the `down` method.

db/migrate/20101026184950_rename_columns_for_devise.rb

@@ -12,7 +12,9 @@ def up
     rename_column :spree_users, :current_login_ip, :current_sign_in_ip
     rename_column :spree_users, :last_login_ip, :last_sign_in_ip
     add_column :spree_users, :authentication_token, :string
-    add_column :spree_users, :unlock_token, :string
+    unless column_exists? :spree_users, :unlock_token
+      add_column :spree_users, :unlock_token, :string
+    end
     add_column :spree_users, :locked_at, :datetime
     remove_column :spree_users, :openid_identifier
   end