Fix part of #15148.

Reproduce issue:
1. Open: http://jsbin.com/zarecedade/3/edit?html,css,output (bootstrap.css from master branch).
2. Make sure that the screen is more "Extra small devices Phones" (>=768px).
3. Open Dropdown 2 (margin-bottom: 2px).

Fix part of #15148.
Please remove this unrelated change.