Fix coach report answer correctness color

Summary:
Addresses issue (https://code.google.com/p/khanacademy/issues/detail?id=23292) / task (http://phabricator.khanacademy.org/T1877)

Maybe at some point the validator just returned true/false; now it returns an object, of which x.correct returns true/false.

Possible Concerns:
Do I need to check that the object exists? I don't think so, because if there is a validator to begin with, it should return an object (as far as I know in Perseus)

I'd push, but I forgot my Github password and was too lazy to reset it.

Test Plan: (1) Check that it's the right color

Reviewers: alex

Reviewed By: alex

Subscribers: alpert, khirasaki

Differential Revision: http://phabricator.khanacademy.org/D7616