Update fabfile
Update machine learning commit hash
Add in coverage requirement
Define custom validator for json fields
add in validation, tests pass for it
Fix validations
Fix validation issue with updating models
Add in some comments for validation
More validations
Add in essay_types and grader_types dicts
Exclude throttle_at field from being changed
Fix existing tests
Remove coverage from main requirements
Merge pull request #62 from edx/dev

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