Detect missing six script

[kshedden]

After my first installation of seaborn, I tried "import seaborn as sns" and got an error:

File "build/bdist.linux-x86_64/egg/seaborn/init.py", line 1, in
File "build/bdist.linux-x86_64/egg/seaborn/rcmod.py", line 5, in
File "build/bdist.linux-x86_64/egg/seaborn/utils.py", line 11, in ImportError: cannot import name range

Checking line 11 of utils.py reveals that this is due to the missing six module. After installing it, everything worked fine. It would be good to detect the missing six module at installation.

[mwaskom]

Ah, sorry, most of the dependencies use six so I figured it would have to be installed for those to work, but they must internally package it.

[mwaskom]

I'm going to close this because packaging six internally is going to be part of the broader changes laid out in #67, which will solve this problem