Poll for the server coming up more frequently

Capybara.timeout sleeps 50ms between polls, and since the server tends
to boot quickly, this is a reasonable value. By not adding another
500ms, we shave a few seconds off the test execution time, mostly in
spec/server_spec.rb.

lib/capybara/server.rb

@@ -66,7 +66,7 @@ def boot
           end
 
           Capybara.timeout(Capybara.server_boot_timeout) do
-            if responsive? then true else sleep(0.5) and false end
+            responsive?
           end
         end
       end