Make StatGraph JS explicitly require its dependencies

This allows us to test them in a bit more isolation.

app/assets/javascripts/stat_graph_contributors.js.coffee

+#= require d3
+#= require jquery
+#= require stat_graph_contributors_util
+
 class @ContributorsStatGraph
   init: (log) ->
     @parsed_log = ContributorsStatGraphUtil.parse_log(log)

app/assets/javascripts/stat_graph_contributors_graph.js.coffee

+#= require d3
+#= require jquery
+#= require underscore
+
 class @ContributorsGraph
   MARGIN:
     top: 20

spec/javascripts/stat_graph_contributors_graph_spec.js

+//= require stat_graph_contributors_graph
+
 describe("ContributorsGraph", function () {
   describe("#set_x_domain", function () {
     it("set the x_domain", function () {

spec/javascripts/stat_graph_contributors_util_spec.js

+//= require stat_graph_contributors_util
+
 describe("ContributorsStatGraphUtil", function () {
 
   describe("#parse_log", function () {

spec/javascripts/stat_graph_spec.js

+//= require stat_graph
+
 describe("StatGraph", function () {
 
   describe("#get_log", function () {