fix overflow on .collapse.in to enable dropdowns within an open collapse

docs/assets/css/bootstrap.css

@@ -1928,6 +1928,7 @@ table .span24 {
 }
 .collapse.in {
   height: auto;
+  overflow: visible;
 }
 .close {
   float: right;

less/component-animations.less

@@ -14,5 +14,8 @@
   position:relative;
   overflow:hidden;
   height: 0;
-  &.in { height: auto; }
+  &.in {
+    height: auto;
+    overflow: visible;
+  }
 }