update d3 dependency handling in demo app

Author: k-rister

tests/package.json

@@ -5,6 +5,7 @@
   "license": "EPL-1.0",
   "dependencies": {
     "axios": "^0.18.0",
+    "d3": "3.5.17",
     "jschart": "file:../jschart",
     "react": "^16.5.1",
     "react-dom": "^16.5.1",

tests/src/App.js

@@ -1,5 +1,6 @@
 import React, { Component } from "react";
 import jschart from "jschart";
+import d3 from "d3";
 
 import histogram_data from './data/histogram.csv';
 import timeseries_data from './data/timeseries.csv';
@@ -8,8 +9,6 @@ import jitter_data from './data/jitter.csv';
 import dynamic_data_1 from './data/ts1.csv';
 import dynamic_data_2 from './data/ts2.csv';
 
-const d3 = require("d3");
-
 var choices = [
   {
     "label": "Null",