Daap: Always send revision-number in update requests (bgo#659146)

Some DAAP servers (e.g. forked-daapd) don't accept update requests
which don't provide a revision-number.

In those cases, Banshee throws a Daap.ContentException with the message:
  Could not find root node 'dmap.serverrevision'

Other DAAP clients (e.g. Rhythmbox) send the first update request with
an initial value of 1 for the revision-number. This patch makes Banshee
have the same behaviour.

Signed-off-by: Bertrand Lorentz <bertrand.lorentz@gmail.com>