Fix broken requests import

Starting to play with balanced and ran into an ImportError right off the bat. :(  This change is a quick fix.  A more robust fix would remove the dependence on an internal, undocumented, unstable variable in another library.

Author: rofrankel

balanced/http_client.py

@@ -2,7 +2,7 @@
 import threading
 
 import requests
-from requests.models import REDIRECT_STATI
+from requests.sessions import REDIRECT_STATI
 
 from balanced import exc
 from balanced.config import Config