Replace future.utils.six with six imports
Add six as a required dependency; update py2/3 doc

Fixes #691.
Merge pull request #696 from jairideout/issue-691

Change future.utils.six imports to direct six imports: redux
Remove skbio deps from readme
Merge pull request #697 from jairideout/dep-doc-rm

Remove skbio deps from readme
Merge branch 'master' into issue-649

Conflicts:
	setup.py
	skbio/draw/_distributions.py