Update cfssl to version 1.6.1 / rev 3 via SR 920662
    
    https://build.opensuse.org/request/show/920662
    by user jfkw + dimstar_suse
    - Update to version 1.6.1:
      * Support for DelegationUsage extension
      * remove -u flag from readme, link to releases
      * remove old go versions from travis
      * fix upgrading transitive coreos dependency breakage (forwarded request 919486 from aplanas)