Fix queue:watcher task to ack messages.

Setting `:ack => true` indicates that messages will be manually acked
(which is a little non-obvious.  This option has been renamed to
`:manual_ack` in newer versions of bunny.)

The default is to auto ack messages, which is what we want in this
instance.
Merge pull request #64 from alphagov/fix_queue_watcher

Fix queue:watcher task to ack messages.