change how collapse works - collapse, collapsing, and in

less/component-animations.less

@@ -16,12 +16,14 @@
 }
 
 .collapse {
+  display: none;
+  &.in {
+    display: block;
+  }
+}
+.collapsing {
   position: relative;
   height: 0;
   overflow: hidden;
   .transition(height .35s ease);
-
-  &.in {
-    height: auto;
-  }
 }