[css] Flex-box: Align last row to grid

Add a ::after which autofills the space. No need to pollute your HTML. Here is a codepen showing it: http://codepen.io/DanAndreasson/pen/ZQXLXj

.grid {
  display: flex;
  flex-flow: row wrap;
  justify-content: space-between;
}

.grid::after {
  content: "";
  flex: auto;
}