refactor stat_graph_contributors_graph to es6 module syntax

app/assets/javascripts/graphs/graphs_bundle.js

-require('./stat_graph_contributors_graph');
 require('./stat_graph_contributors');

app/assets/javascripts/graphs/stat_graph_contributors.js

 /* eslint-disable func-names, space-before-function-paren, wrap-iife, no-var, one-var, camelcase, one-var-declaration-per-line, quotes, no-param-reassign, quote-props, comma-dangle, prefer-template, max-len, no-return-assign */
-/* global ContributorsGraph */
-/* global ContributorsAuthorGraph */
-/* global ContributorsMasterGraph */
+
+import { ContributorsGraph, ContributorsAuthorGraph, ContributorsMasterGraph } from './stat_graph_contributors_graph';
 import ContributorsStatGraphUtil from './stat_graph_contributors_util';
 
 /* global d3 */

spec/javascripts/graphs/stat_graph_contributors_graph_spec.js

-/* eslint-disable quotes, jasmine/no-suite-dupes, vars-on-top, no-var, max-len */
-/* global d3 */
-/* global ContributorsGraph */
-/* global ContributorsMasterGraph */
+/* eslint-disable quotes, jasmine/no-suite-dupes, vars-on-top, no-var */
 
-require('~/graphs/stat_graph_contributors_graph');
+import d3 from 'd3';
+import { ContributorsGraph, ContributorsMasterGraph } from '~/graphs/stat_graph_contributors_graph';
 
 describe("ContributorsGraph", function () {
   describe("#set_x_domain", function () {