Support headless firefox screenshots

This was probably just an ommission. It's quite useful to get
screenshots when running failing headless specs.

Author: mamhoff

lib/capybara-screenshot.rb

@@ -161,6 +161,7 @@ def self.capybara_tmp_path=(path)
   end
 
   register_driver :selenium, &selenium_block
+  register_driver :selenium_headless, &selenium_block
   register_driver :selenium_chrome, &selenium_block
   register_driver :selenium_chrome_headless, &selenium_block