Remove the margin-bottom from <label>s in .btn-groups (twbs#20827)

- Fixes twbs#20298 for vertical button group spacing
- Fixes twbs#20784 for horizontal (default) button group spacing
- Alternate fix to twbs#20823 which only applied a fix for vertical button groups

Author: mdo

scss/_button-group.scss

@@ -10,6 +10,7 @@
   > .btn {
     position: relative;
     float: left;
+    margin-bottom: 0;
 
     // Bring the "active" button to the front
     &:focus,