exclude cmd2 0.8.3 and update to 0.8.4
exclude cmd2 0.8.3 and update to 0.8.4

Release 0.8.3 of cmd2 failed to properly specify a new dependency so
installing it breaks interactive mode for OSC and other cliff-based
apps. 0.8.4 fixes the issue.

Change-Id: I8bbd749e3328596fd333bd7f58391b9d688c00cf
Depends-On: https://review.openstack.org/560120
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

openstack/cliff,dtroyer/cliff,dtroyer/cliff,openstack/cliff