Fix "I don't know this button" for mastery tasks

Summary:
Prior to this change, clicking "I don't know this" button sent up the
request to the server for the *following* exercise.

Test Plan:
Open a mastery task
in JS console, check exercise name via Exercises.currentCard.attributes.exerciseName
Click "I don't know this", verify in Network tab that XHR is for the right exercise
Do the same for an assessment task

Reviewers: james, mattdunnrankin, alpert

Reviewed By: alpert

CC: marcia, alpert

Differential Revision: http://phabricator.khanacademy.org/D3026