Replace deprecated method

Change-Id: I1b79c172d032d96651d5b5c36cbf0c662081da28

tests/browser/features/step_definitions/persistent_steps.rb

@@ -35,5 +35,5 @@ end
 
 Then(/^I should see the text in the language panel in (.+?)$/) do |language|
 	code = language_to_code(language)
-	on(PanelPage).uls_display_settings_element.attribute_value('lang').should == code
+	on(PanelPage).uls_display_settings_element.attribute('lang').should == code
 end