fix vertical button-groups in ie7

docs/assets/css/bootstrap.css

@@ -3516,6 +3516,10 @@ input[type="submit"].btn.btn-mini {
 
 .btn-group-vertical {
   display: inline-block;
+  *display: inline;
+  /* IE7 inline-block hack */
+
+  *zoom: 1;
 }
 
 .btn-group-vertical .btn {

less/button-groups.less

@@ -214,6 +214,7 @@
 
 .btn-group-vertical {
   display: inline-block; // makes buttons only take up the width they need
+  .ie7-inline-block();
 }
 .btn-group-vertical .btn {
   display: block;