Centered columns incorrectly reset margins when in mobile display

All margins should not be resetted, only the `margin-left` (used to center the column) should. Here are the visual modifications in [the documentation](http://gumbyframework.com/docs/grid/#!/centered-columns):

**Current behavior**
![before](https://f.cloud.github.com/assets/838867/742169/264d6e34-e3c2-11e2-93bb-1618394665a5.png)

**Expected behavior**
![after](https://f.cloud.github.com/assets/838867/742170/294109e8-e3c2-11e2-8c36-a2647ecccaf9.png)