Import get_user_model from compat module

Import get_user_model from compat module to be compatible with older django versions (e.g. 1.4).

rest_framework/authentication.py

@@ -4,12 +4,11 @@ Provides various authentication policies.
 from __future__ import unicode_literals
 import base64
 from django.contrib.auth import authenticate
-from django.contrib.auth import get_user_model
 from django.middleware.csrf import CsrfViewMiddleware
 from django.utils.translation import ugettext_lazy as _
 from rest_framework import exceptions, HTTP_HEADER_ENCODING
 from rest_framework.authtoken.models import Token
-
+from rest_framework.compat import get_user_model
 
 def get_authorization_header(request):
     """