Drop support for Django 1.3, Python 2.6 and 3.2.

* Shrink the Travis test matrix
* Remove a lot of backported test code
* Update requirements in setup.py
* Update docs

Closes #79

djrill/tests/mock_backend.py

@@ -4,13 +4,12 @@ import requests
 import six
 
 from django.test import TestCase
-
-from .utils import BackportedAssertions, override_settings
+from django.test.utils import override_settings
 
 
 @override_settings(MANDRILL_API_KEY="FAKE_API_KEY_FOR_TESTING",
                    EMAIL_BACKEND="djrill.mail.backends.djrill.DjrillBackend")
-class DjrillBackendMockAPITestCase(TestCase, BackportedAssertions):
+class DjrillBackendMockAPITestCase(TestCase):
     """TestCase that uses Djrill EmailBackend with a mocked Mandrill API"""
 
     class MockResponse(requests.Response):