Added missing dependency: six.

Author: jonathanslenders

setup.py

@@ -18,6 +18,7 @@
     packages=find_packages('.'),
     install_requires = [
         'prompt_toolkit>=2.0.0,<3.1.0',
+        'six',
         'pyflakes',        # For Python error reporting.
         'pygments',        # For the syntax highlighting.
         'docopt',          # For command line arguments.