Updated StringIO import to support io module

Signed-off-by: Tony Witherspoon <[email protected]>

tests/unit/cli_test.py

@@ -5,7 +5,7 @@ from __future__ import unicode_literals
 import os
 import shutil
 import tempfile
-from StringIO import StringIO
+from io import StringIO
 
 import docker
 import py