Slickカルーセルのアイテム間にスペースを入れる方法

    $('.single-item').slick({
        initialSlide: 3,
        infinite: false
    });

.slick-slider {
    margin:0 -15px;
}
.slick-slide {
    padding:10px;
    background-color:red;
    text-align:center;
    margin-right:15px;
    margin-left:15px;
}

<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/js/bootstrap.min.js"></script>
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css" rel="stylesheet"/>
<script src="http://cdn.jsdelivr.net/jquery.slick/1.5.0/slick.min.js"></script>
<link href="http://cdn.jsdelivr.net/jquery.slick/1.5.0/slick.css" rel="stylesheet"/>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
<div class="container">
    <div class="row">
        <div class="col-sm-12" style="background-color:gray;">
            <div class="slider single-item" style="background:yellow">
                <div>1</div>
                <div>2</div>
                <div>3</div>
                <div>4</div>
                <div>5</div>
                <div>6</div>
            </div>
        </div>
    </div>
</div>