Use unittest2 for rhel6 compatibility

Fixes: https://pagure.io/koji/issue/830

tests/test_plugins/test_runroot_cli.py

@@ -1,7 +1,10 @@
 from __future__ import absolute_import
 import mock
 import six
-import unittest
+try:
+    import unittest2 as unittest
+except ImportError:
+    import unittest
 
 import koji
 from . import load_plugin