Final code

<!DOCTYPE html>
<html>
<head>
    <meta charset='utf-8' />
    <title></title>
    <meta name='viewport' content='initial-scale=1,maximum-scale=1,user-scalable=no' />
    <script src='https://api.tiles.mapbox.com/mapbox-gl-js/v0.39.1/mapbox-gl.js'></script>
    <link href='https://api.tiles.mapbox.com/mapbox-gl-js/v0.39.1/mapbox-gl.css' rel='stylesheet' />
    <style>
        body { margin:0; padding:0; }
        #map { position:absolute; top:0; bottom:0; width:100%; }
    </style>
</head>
<body>

.map-overlay .map-overlay-inner {
    background-color: #fff;
    box-shadow:0 1px 2px rgba(0, 0, 0, 0.20);
    border-radius: 3px;
    padding: 10px;
    margin-bottom: 10px;
}

.map-overlay h2 {
    line-height: 24px;
    display: block;
    margin: 0 0 10px;
}

.map-overlay .legend .bar {
    height: 10px;
    width: 100%;
    background: linear-gradient(to right, #FCA107, #7F3121);
}

.map-overlay input {
    background-color: transparent;
    display: inline-block;
    width: 100%;
    position: relative;
    margin: 0;
    cursor: ew-resize;
}
</style>

<div class='map-overlay top'>
    <div class='map-overlay-inner'>
        <h2>Significant earthquakes in 2015</h2>
        <label id='month'></label>
        <input id='slider' type='range' min='0' max='11' step='1' value='0' />
    </div>
    <div class='map-overlay-inner'>
        <div id='legend' class='legend'>
            <div class='bar'></div>
            <div>Magnitude (m)</div>
        </div>
    </div>
</div>

var months = [
    'January',
    'February',
    'March',
    'April',
    'May',
    'June',
    'July',
    'August',
    'September',
    'October',
    'November',
    'December'
];

function filterBy(month) {

var filters = ['==', 'month', month];
    map.setFilter('earthquake-circles', filters);

// Set the label to the month
    document.getElementById('month').textContent = months[month];
}

map.on('load', function() {

// Data courtesy of https://earthquake.usgs.gov/
    // Query for significant earthquakes in 2015 URL request looked like this:
    // https://earthquake.usgs.gov/fdsnws/event/1/query
    //    ?format=geojson
    //    &starttime=2015-01-01
    //    &endtime=2015-12-31
    //    &minmagnitude=6'
    //
    // Here we're using d3 to help us make the ajax request but you can use
    // Any request method (library or otherwise) you wish.
    d3.json('https://www.mapbox.com/mapbox-gl-js/assets/data/significant-earthquakes-2015.geojson', function(err, data) {
        if (err) throw err;

// Create a month property value based on time
        // used to filter against.
        data.features = data.features.map(function(d) {
            d.properties.month = new Date(d.properties.time).getMonth();
            return d;
        });

map.addSource('earthquakes', {
            'type': 'geojson',
            'data': data
        });

map.addLayer({
            'id': 'earthquake-circles',
            'type': 'circle',
            'source': 'earthquakes',
            'paint': {
                'circle-color': {
                    property: 'mag', //set property
                    stops: [
                        [6, '#FCA107'],
                        [8, '#7F3121']
                    ]
                },
                'circle-opacity': 1,
                'circle-radius': 20
            }
        });

// Set filter to first month of the year
        // 0 = January
        filterBy(0);

document.getElementById('slider').addEventListener('input', function(e) {
            var month = parseInt(e.target.value, 10);
            filterBy(month);
        });
    });
});
</script>

</body>
</html>

https://api.tiles.mapbox.com/mapbox-gl-js/v0.39.1/mapbox-gl.js

https://d3js.org/d3.v3.min.js