Build docs in Travis

And clean up requirements files
Remove workflow step validity check (to support rollbacks)

Refactor workflow API to avoid hard-coding first step logic.
Merge pull request #420 from singingwolfboy/db/build-docs-in-travis

Build docs in Travis
Merge pull request #423 from edx/will/more-workflow-refactoring

More workflow refactoring
Update test call in README
Merge pull request #425 from edx/will/update-test-example-readme

Update test call in README
Merge remote-tracking branch 'origin/master' into ai-grading

Conflicts:
	README.rst
	openassessment/assessment/api/peer.py
	openassessment/workflow/api.py
	openassessment/workflow/models.py
	openassessment/workflow/test/test_api.py