Try to fix the build for rails 3.1

lib/devise/failure_app.rb

@@ -88,7 +88,9 @@ module Devise
       opts  = {}
       route = :"new_#{scope}_session_path"
       opts[:format] = request_format unless skip_format?
-      opts[:script_name] = Rails.application.config.relative_url_root
+
+      config = Rails.application.config
+      opts[:script_name] = (config.relative_url_root if config.respond_to?(:relative_url_root))
 
       context = send(Devise.available_router_name)
 

test/failure_app_test.rb

@@ -51,11 +51,13 @@ class FailureTest < ActiveSupport::TestCase
       end
     end
 
-    test 'returns to the default redirect location considering the relative url root' do
-      swap Rails.application.config, :relative_url_root => "/sample" do
-        call_failure
-        assert_equal 302, @response.first
-        assert_equal 'http://test.host/sample/users/sign_in', @response.second['Location']
+    if Rails.application.config.respond_to?(:relative_url_root)
+      test 'returns to the default redirect location considering the relative url root' do
+        swap Rails.application.config, :relative_url_root => "/sample" do
+          call_failure
+          assert_equal 302, @response.first
+          assert_equal 'http://test.host/sample/users/sign_in', @response.second['Location']
+        end
       end
     end