Proper border for justified btn-groups inside tables

Fixes #9406.

This fiddle shows the issue with the v3 CSS _before_ my fix (note that the border on the corners isn't rounded, and when hovering the button on the right the "shared edge" doesn't get darker):
http://jsfiddle.net/MgcDU/6647/

This is the same fiddle with the new CSS (both issues disappeared):
http://jsfiddle.net/MgcDU/6648/

The only change was setting `border-collapse: separate` on `.btn-group-justified`, to avoid it inheriting the `border-collapse: collapse` of the `.table` it's placed inside.