Fix Python 2.6

tests/__init__.py

@@ -0,0 +1,7 @@
+import sys
+
+if sys.version_info >= (2,7):
+    import unittest
+else:
+    import unittest2 as unittest
+

tests/testcases.py

@@ -1,10 +1,10 @@
 from docker import Client
 from fig.service import Service
 from fig.cli.utils import docker_url
-from unittest2 import TestCase
+from . import unittest
 
 
-class DockerClientTestCase(TestCase):
+class DockerClientTestCase(unittest.TestCase):
     @classmethod
     def setUpClass(cls):
         cls.client = Client(docker_url())