Set 'server' parameter on GTalk accounts.

This stops us depending on properly-configured SRV records.
'fallback-servers' is not good enough in this situation:

 • foo.com's XMPP stuff is served by Google Apps;
 • your router breaks SRV requests;
 • foo.com resolves to 1.2.3.4 which has a (different) XMPP server
   listening on port 5222.

This may seem far-fetched, but this happened to two completely different
people, both hosted with Dreamhost.

https://bugzilla.gnome.org/show_bug.cgi?id=653510