setup.py: Don't pin exact version of six

Since furl is a library, having furl pin exact versions of dependencies means you are limiting the usability of furl when you pin an exact version of `six`. Six months down the road, someone may have an app and want to use `six>=1.9.0` or even `six==1.8.2` (Or a library the app uses may require this) and then they'll be stuck. Best to keep dependencies as loose as possible. If furl works with `six==1.6.0` for example then change it to require `six>=1.6` and more people will be able to use furl.