make sure only valid drivers take screenshots

lib/cuke-pack/support/screenshots.rb

@@ -7,9 +7,11 @@ module CukePack
 end
 
 def take_screenshot(name, options = {})
+  selenium = Capybara.current_session.driver.browser
+
+  if selenium.respond_to?(:manage)
     options = CukePack.screenshot_options.merge(options)
 
-  selenium = Capybara.current_session.driver.browser
     selenium.manage.window.resize_to(options[:width], options[:height])
 
     target = options[:directory]
@@ -19,4 +21,5 @@ def take_screenshot(name, options = {})
     FileUtils.mkdir_p File.dirname(target)
 
     selenium.save_screenshot(target)
+  end
 end