Switch taskId back to task_id

Summary:
The client was changed from task_id to taskId in http://phabricator.khanacademy.org/D8297,
the server was changed to accept both task_id or taskId in http://phabricator.khanacademy.org/rGWcde63d08f3b2e03b4234e5b83201610cfaa8cb2a

And now we're changing the client from taskId back to the original task_id!
In a short while, I'll change the server back to only accept task_id, as before.

Going to add a Perseus person in case this affects you in some way! Lucky Jack!

Reviewers: mgp, jack, benkomalo, laura

Reviewed By: benkomalo, laura

Differential Revision: http://phabricator.khanacademy.org/D8423