Bikeshed update for 'Remove 'auto' value for ScrollDirection (#32)

As specced this value causes implementation difficulties around the
definition of 'scrollable' and the possibility of instability in the
resultant ScrollTimeline (see issues #15 and #16). For now we have
resolved to just remove it, with "block" becoming the default for a
ScrollTimeline.' [ci skip]

Generated from:

commit f566763
Author: Stephen McGruer <stephen.mcgruer@gmail.com>
Date:   Wed Nov 21 13:21:43 2018 -0500

    Remove 'auto' value for ScrollDirection (#32)

    As specced this value causes implementation difficulties around the
    definition of 'scrollable' and the possibility of instability in the
    resultant ScrollTimeline (see issues #15 and #16). For now we have
    resolved to just remove it, with "block" becoming the default for a
    ScrollTimeline.

Author: stephenmcgruer