Add try/catch for simplejson vs json

requests/models.py

@@ -7,10 +7,14 @@ requests.models
 This module contains the primary objects that power Requests.
 """
 
-import json
 import os
 from datetime import datetime
 
+try:
+    import simplejson as json
+except ImportError:
+    import json
+
 from .hooks import dispatch_hook, HOOKS
 from .structures import CaseInsensitiveDict
 from .status_codes import codes