jquery.ajaxdispatcher: Don't use 'new' to create a deferred

Despite it's uppercaseness, $.Deferred is not a constructor.
It's an object factory. Adding 'new' is harmless, but confusing
and redundant. It'd be like invoking any other function that
returns an object with 'new', like `new mw.user.getRights()`.

Change-Id: I7db7340c5f324cbbc59d298c9afd6dc1866eb1d8