Fix Python 3.2 compatibility with the help of six.u

Update tox to test in Python 2.5 and above, now that those versions are
all confirmed to work.

tests/test_api_transaction.py

@@ -1,4 +1,4 @@
-from six import BytesIO
+from six import BytesIO, u
 from datetime import date
 
 import mock
@@ -100,7 +100,7 @@ def test_make_call(self, urlopen):
     @mock.patch('authorize.apis.transaction.urlopen')
     def test_make_call_with_unicode(self, urlopen):
         urlopen.side_effect = self.success
-        result = self.api._make_call({u'\xe3': '1', 'b': u'\xe3'})
+        result = self.api._make_call({u('\xe3'): '1', 'b': u('\xe3')})
         self.assertEqual(URL(urlopen.call_args[0][0]),
             URL('{0}?%C3%A3=1&b=%C3%A3'.format(TEST_URL)))
         self.assertEqual(result, PARSED_SUCCESS)

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist=py26,py27,py34
+envlist=py25,py26,py27,py31,py32,py33,py34
 [testenv]
 deps = -rrequirements.txt
 passenv=AUTHORIZE_LIVE_TESTS