reverts changes for loading mapnik-reference when is actually needed

lib/carto/tree/reference.js

@@ -5,6 +5,7 @@
 (function(tree) {
 
 var _ = require('underscore'),
+    mapnik_reference = require('mapnik-reference'),
     ref = {};
 
 ref.setData = function(data) {
@@ -23,7 +24,6 @@ ref.setData = function(data) {
 };
 
 ref.setVersion = function(version) {
-    var mapnik_reference = require('mapnik-reference');
     if (mapnik_reference.version.hasOwnProperty(version)) {
         ref.setData(mapnik_reference.version[version]);
         return true;