Sort data by date when building graphs

This doesn't affect the end result because all collections used the same
order, but it makes debugging easier.

spec/models/ahoy/data_source_spec.rb

@@ -25,5 +25,15 @@ describe Ahoy::DataSource do
                              "foo" => [2, 1, 0],
                              "bar" => [1, 0, 2]
     end
+
+    it "returns data ordered by dates" do
+      ds = Ahoy::DataSource.new
+      ds.add "foo", { january_third => 2, january_second => 1 }
+      ds.add "bar", { january_first => 2, january_second => 1 }
+
+      expect(ds.build).to eq :x => ["2015-01-01", "2015-01-02", "2015-01-03"],
+                             "foo" => [0, 1, 2],
+                             "bar" => [2, 1, 0]
+    end
   end
 end