Change the directory of Bootstrap from '_assets' to static 'assets'.

assets/vendor/bootstrap/less/component-animations.less

@@ -0,0 +1,29 @@
+//
+// Component animations
+// --------------------------------------------------
+
+// Heads up!
+//
+// We don't use the `.opacity()` mixin here since it causes a bug with text
+// fields in IE7-8. Source: https://github.com/twitter/bootstrap/pull/3552.
+
+.fade {
+  opacity: 0;
+  .transition(opacity .15s linear);
+  &.in {
+    opacity: 1;
+  }
+}
+
+.collapse {
+  display: none;
+  &.in {
+    display: block;
+  }
+}
+.collapsing {
+  position: relative;
+  height: 0;
+  overflow: hidden;
+  .transition(height .35s ease);
+}