Add API calls for checking workflow completeness
Skip grading/training tasks if the workflow is already complete.
Fallback to file-based cache for classifier data
Add info-level logging for cache hits/misses
Catch HTTP errors that occur when downloading classifier data
Bugfix: validate duplicate point values for rubrics with example-based assessments
Concatenate validation errors with semicolons instead of newlines so they show up correctly in Studio
Merge pull request #454 from edx/will/ai-skip-completed-workflows

Skip completed workflows
Merge pull request #458 from edx/will/TIM-660

Bugfix: validate duplicate point values for rubrics with example-based assessments
Merge pull request #457 from edx/will/TIM-661

Catch HTTP errors that occur when downloading classifier data
Merge pull request #459 from edx/will/TIM-659

Concatenate validation errors with semicolons instead of newlines so they show up correctly in Studio
Merge pull request #456 from edx/will/ai-cache-classifiers-to-disk

AI: Celery workers lose their cache after an exception causing many hits to S3
Changed NLTK requirements from 2.0.3 to 2.0.4
Update wheel archive for NLTK