stop relying on ujson (fix #589, #587, #507)

Author: ziirish

flask_restplus/representations.py

@@ -1,10 +1,7 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals, absolute_import
 
-try:
-    from ujson import dumps
-except ImportError:
-    from json import dumps
+from json import dumps
 
 from flask import make_response, current_app