Make sure that exit codes of tests are propagated properly when using selenium webdriver.

lib/capybara/selenium/driver.rb

@@ -16,7 +16,10 @@ class Capybara::Selenium::Driver < Capybara::Driver::Base
 
       main = Process.pid
       at_exit do
+        # Store the exit status of the test run since it goes away after calling the at_exit proc...
+        @exit_status = $!.status if $!.is_a?(SystemExit)
         quit if Process.pid == main
+        exit @exit_status if @exit_status # Force exit with stored status
       end
     end
     @browser