Fixes bottom attached message bottom margin.

Example & test case: http://jsfiddle.net/psycketom/w5BPQ/

Currently, a message that is "bottom attached" will not leave margin behind, making following inline elements, like button, be undivided from it.

This attempts to fix it.

The `:not(:last-child)` filter is applied, because `.ui.segment > :last-child` was overridden by the more precise selector (or maybe even selector order inside prepacked build). I obeyed applying `!important` to latter selector because it's simply discouraged.

While images are nice to have for PR's, from now on I will rely only on Example & Test Case fiddles... mainly, due to my laziness.

P.S. This fix extends upon #568. The test case, though, is tuned for 0.12.2, yet, remains forward compatible to #568.