Fix test fails on Python 2.6

TestCase.assertIsNone() is in Python 2.7+ and Python 3.1+

Author: methane

test/test_msgpackrpc.py

@@ -1,5 +1,8 @@
 import threading
-import unittest
+try:
+    import unittest2 as unittest
+except ImportError:
+    import unittest
 
 import helper
 import msgpackrpc

tox.ini

@@ -5,3 +5,9 @@ deps=
     pytest
     six
 commands=py.test test
+[testenv:py26]
+deps=
+    pytest
+    six
+    unittest2
+commands=py.test test