require in new trace modules,

 - so that the new trace modules are registered via Plots.register

Author: etpinard

src/plotly.js

@@ -61,6 +61,8 @@ exports.Bars = require('./traces/bars');
 exports.Boxes = require('./traces/boxes');
 exports.Heatmap = require('./traces/heatmap');
 exports.Histogram = require('./traces/histogram');
+exports.Histogram2d = require('./traces/histogram2d');
+exports.Histogram2dContour = require('./traces/histogram2dcontour');
 exports.Pie = require('./traces/pie');
 exports.Contour = require('./traces/contour');
 exports.Scatter3D = require('./traces/scatter3d');