generate sourcemap for register and save it

lib/6to5/register.js

@@ -13,10 +13,10 @@ require.extensions[".es6"] = function (m, filename) {
   }
 
   var result = node.transformFileSync(filename, {
-    sourceMapObject: true
+    sourceMap: true
   });
 
-  //maps[filename] = result.map;
+  maps[filename] = result.map;
 
   m._compile(result.code, filename);
 };