fixes #13375

Add selectors to remove the bottom margin from tables that are inside a
.panel-collapse div

When using a table inside a collapse component, the margin-bottom of the table isn't set to 0 (as it is when using a table directly inside a panel) causing a ugly white space to appear below the table.

**Test case on JSBin:**
http://jsbin.com/koretoje/2/

Fixes #13375.

This is my first pull request, FYI.