[cli] use 'avg' api from python-progress
    
    This has been fixed in f24+ and epel7 (rhbz#1299634).  The benefit
    of official API is that it implements "moving average" speed
    measurement (the real speed for latest several seconds), instead
    of average download speed for the whole download time.
    
    This reverts commit fc498e28d1c87d304d0e2c75798ca502e066f0a8.