[css3-flexbox] flex-pack distribute bikeshed

I understand the desire to use distribute for flex-pack since it can be
vertical as well, but I think justify is a more well understood term and I
don't foresee people finding the concept of justifying vertically confusing.
The meaning is obvious.

WebKit already implemented it with 'justify' based on the previous state of
the spec and I'm loathe to change the name to something that's IMO more
confusing.