This package requires emacs24
-----------------------------

Please note that async.el is not compatible with Emacs 23.  If
/usr/bin/emacs points to emacs23, you must run

    # apt-get install emacs24
    # update-alternatives --config emacs

and select emacs24 in order to be able to use async.el without any
errors.

 -- Sean Whitton <spwhitton@spwhitton.name>, Wed, 13 Jan 2016 08:59:39 -0700