Keep only the last revision of content with multi-version module
Actually, now (March 30 2018) it has a 8.x version. So this is your module:

Node Revision Delete


  The Node Revision Delete module lets you to track and prune old
  revisions of content types.
  
  Features:
  
  
  Define the max amount of revisions to keep per content type.
  Run on Drush, cron run or on a different period (daily, weekly, etc).