_semantic-grid.scss: changed how margins are set for centered column

No longer using !important; no longer setting top and bottom values.

Reasoning: 
1) If developers currently want to add top/bottom margins to a centered column, they have to use !important, to override the semantic grid's !important. This could result in lots of !importants.

2) If developers want to adjust left/right margins of a centered column - for example, if the design changes at a higher breakpoint - they would have to use !important, to override the semantic grid's !important. This could result in lots of !importants.