refactor stat_graph_contributors_util to es6 module syntax

app/assets/javascripts/graphs/graphs_bundle.js

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

app/assets/javascripts/graphs/stat_graph_contributors.js

@@ -2,7 +2,8 @@
 /* global ContributorsGraph */
 /* global ContributorsAuthorGraph */
 /* global ContributorsMasterGraph */
-/* global ContributorsStatGraphUtil */
+import ContributorsStatGraphUtil from './stat_graph_contributors_util';
+
 /* global d3 */
 
 window.d3 = require('d3');

app/assets/javascripts/graphs/stat_graph_contributors_util.js

 /* eslint-disable func-names, space-before-function-paren, object-shorthand, no-var, one-var, camelcase, one-var-declaration-per-line, comma-dangle, no-param-reassign, no-return-assign, quotes, prefer-arrow-callback, wrap-iife, consistent-return, no-unused-vars, max-len, no-cond-assign, no-else-return, max-len */
-(function() {
-  window.ContributorsStatGraphUtil = {
+
+export default {
   parse_log: function(log) {
     var by_author, by_email, data, entry, i, len, total, normalized_email;
     total = {};
@@ -134,5 +134,4 @@
       return false;
     }
   }
-  };
-}).call(window);
+};

spec/javascripts/graphs/stat_graph_contributors_util_spec.js

 /* eslint-disable quotes, no-var, camelcase, object-property-newline, comma-dangle, max-len, vars-on-top, quote-props */
-/* global ContributorsStatGraphUtil */
 
-require('~/graphs/stat_graph_contributors_util');
+import ContributorsStatGraphUtil from '~/graphs/stat_graph_contributors_util';
 
 describe("ContributorsStatGraphUtil", function () {
   describe("#parse_log", function () {