Change future.utils.six imports to direct six imports: redux

My previous PR (#695) wasn't being noticed by Travis, so resubmitting:

Added `six` as a required dependency and changed all `future.utils.six` imports to direct `six` imports. Also updated py2/3 guide.

As it turns out, matplotlib already required `six`, so this was being installed as an indirect skbio dependency.

Fixes #691.