update histogram2dcontour for contour.plot API change

Author: alexcjohnson

src/traces/histogram2dcontour/index.js

@@ -14,7 +14,7 @@ var Histogram2dContour = {};
 Histogram2dContour.attributes = require('./attributes');
 Histogram2dContour.supplyDefaults = require('./defaults');
 Histogram2dContour.calc = require('../contour/calc');
-Histogram2dContour.plot = require('../contour/plot');
+Histogram2dContour.plot = require('../contour/plot').plot;
 Histogram2dContour.style = require('../contour/style');
 Histogram2dContour.colorbar = require('../contour/colorbar');
 Histogram2dContour.hoverPoints = require('../contour/hover');