Missing package six in the install requirements

Hi,

Sorry if it is not the right / complete way to fix it, I am not yet used to the python package system.

When I installed zipline the first time and tried to execute the demo, I got the error "ImportError: No module named six".

Indeed, the package is imported here :
https://github.com/quantopian/zipline/blob/master/zipline/data/loader.py#L29

Thanks !