Basic CartoDB + D3 Choropleth

<!DOCTYPE html>
<html>
  <head>
    <meta http-equiv="Content-Type" content="text/html;charset=utf-8">
    <title>Choropleth</title>
    <script src="https://d3js.org/d3.v3.min.js"></script>
    <script src="https://d3js.org/d3.geo.projection.v0.min.js"></script>
    <style type="text/css">

body{
        background:white;
    }
    svg {
      width: 960px;
      height: 500px;
      background: none;
    }
    svg:active {
      cursor: move;
      cursor: -moz-grabbing;
      cursor: -webkit-grabbing;
    }
    .globe {
      fill: black;
      fill-opacity: 1;
      stroke: #111;
      stroke-width:1px;
    }
    #first_layer path {
      stroke: #333;
      stroke-linecap: round;
      stroke-linejoin: round;
    }
    </style>
  </head>
  <body>
    <script type="text/javascript">

// Define our SVG element outside our SQL functions
    var svg = d3.select("body")
            .append("svg")
            .call(d3.behavior.zoom()
                .on("zoom", redraw))
            .append("g");

// Define the color scale for our choropleth
    var fill = d3.scale.linear()
        .domain([0, 1000, 10000000, 100000000, 1000000000])
        .range(["#333", "#6ab9fa", "#39a2f8", "#098bf5", "#0666b4"]);

// Our projection.
    var xy = d3.geo.robinson();

// Our path
    var path = d3.geo.path()
                .projection(xy);

// Using the graticule to draw the globe background color
    var graticule = d3.geo.graticule();
    svg.append("path")
        .datum(graticule.outline)
        .attr("class", "globe")
        .attr("d", path);

// Add our first layer
    svg.append("g").attr("id", "first_layer");

// Use D3 AJAX method to query CartoDB table
    d3.json("https://viz2.cartodb.com/api/v2/sql?q=SELECT ST_Simplify(the_geom,0.01) as the_geom, pop2005 as population FROM d3_world_borders WHERE the_geom IS NOT NULL&format=geojson&dp=5", function(collection) {
          svg.select("#first_layer")
            .selectAll("path")
              .data(collection.features)
            .enter().append("path")
            .attr("fill", function(d) { 
              // Return a color from our color ramp based on pop size
              return fill(d.properties.population); 
             })
            .attr("stroke-width", "0.5px")
            .attr("fill-opacity", "0.7")
            .on("mouseover", function(t,d){
              // Mouseover, remove transparency
              d3.select(this)
                .attr("stroke-width", "1px")
                .attr("fill-opacity", "0.9");
              })
            .on("mouseout", function(t,d){
              // Mouseover, replace transparency
              d3.select(this)
                .attr("stroke-width", "0.5px")
                .attr("fill-opacity", "0.7");
              })
            .attr("d", path.projection(xy));
      })
    // Handles our zoom
    function redraw() {
      svg.attr("transform", "translate(" + d3.event.translate + ")scale(" + d3.event.scale + ")");
    }

</script>
  </body>
</html>

Modified http://d3js.org/d3.v3.min.js to a secure url

Modified http://d3js.org/d3.geo.projection.v0.min.js to a secure url

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

https://d3js.org/d3.geo.projection.v0.min.js