add missing install requirement of six
diff --git a/setup.cfg b/setup.cfg
index b3e33a9..57c62a1 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -29,6 +29,7 @@
     testing, test, mock, mocking, unittest, patching, stubs, fakes, doubles
 
 [options]
+install_requires=six
 python_requires=>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*
 packages = mock