use unittest2 so python 2.6 doesn't blow up

Author: mikesea

test_requirements.txt

@@ -1,3 +1,4 @@
 nose==1.3.0
 vcrpy==0.5.0
 mock>=1.0.1
+unittest2

tests/test_swiftype.py

@@ -1,7 +1,7 @@
 from swiftype import swiftype
 import os
 import time
-import unittest
+import unittest2 as unittest
 import vcr
 from mock import Mock