ARROW-349: Add six as a requirement

fixes https://issues.apache.org/jira/browse/ARROW-349

Author: Peter Hoffmann <ph@peter-hoffmann.com>

Closes #184 from hoffmann/patch-1 and squashes the following commits:

1bffc69 [Peter Hoffmann] Add six as a requirement

Author: hoffmann

python/setup.py

@@ -271,7 +271,7 @@ def get_outputs(self):
         'clean': clean,
         'build_ext': build_ext
     },
-    install_requires=['cython >= 0.23', 'numpy >= 1.9'],
+    install_requires=['cython >= 0.23', 'numpy >= 1.9', 'six >= 1.0.0'],
     description=DESC,
     license='Apache License, Version 2.0',
     maintainer="Apache Arrow Developers",