prosody: update to 0.9.7

update prosody to 0.9.7 to get the latest fixes. Changelogs:
http://blog.prosody.im/prosody-0-9-5-released/
http://blog.prosody.im/prosody-0-9-6-released/
http://blog.prosody.im/prosody-0-9-7-released/

Tested for a few days with a dozen clients

Signed-off-by: Stefan Hellermann <stefan@the2masters.de>