Fix tests on python-2.6

tests/__init__.py

@@ -2,5 +2,7 @@
 import sys
 if sys.version_info < (2, 7):
 	from unittest2 import TestCase, main  # NOQA
+	from unittest2.case import SkipTest  # NOQA
 else:
 	from unittest import TestCase, main  # NOQA
+	from unittest.case import SkipTest  # NOQA

tests/test_lib.py

@@ -5,9 +5,8 @@ from powerline.lib.vcs import guess
 from subprocess import call, PIPE
 import os
 import sys
-from unittest.case import SkipTest
 from functools import partial
-from tests import TestCase
+from tests import TestCase, SkipTest
 
 
 class TestLib(TestCase):