I4bb02749a8fafccaff4ef7ce94ff0c4910fb67e6 fixed cmd2 0.7.0 but
broke <=0.7.0. Detect if Cmd.do_hi() is present to decide what
the response should be to support both cases.
This can be reverted some time after cmd2 0.7.0 is in cliff's
requirements.txt as the minimum version.
Change-Id: I3d42aa68303bed1c302e72d1eab861d355646bc7