prosody: update to 0.11.13

From the release notes:

  This is a(nother!) release for our stable branch to fix a memory leak caused
  by the security fix. Deployments using websockets, SQL storage and possibly
  other configurations may have noticed increasing memory usage after upgrading
  to 0.11.12. This is resolved by this new release.

  Minor changes:

  * util.xml: Break reference to help the GC (fixes #1711)
  * util.xml: Deduplicate handlers for restricted XML

(khorben)