.collapse.in {
     height: auto; 
}

.collapse {
    -webkit-transition: height 0.35s ease;
    -moz-transition: height 0.35s ease;
    -ms-transition: height 0.35s ease;
    -o-transition: height 0.35s ease;
    transition: height 0.35s ease;
    position: relative;
    overflow: hidden;
    height: 0;

}