Hack to fix attribute order in clean-exercises.py.

Summary:
class and id should come before all the other attributes, even though d comes
before i alphabetically.

Test Plan: Ran it against exercises/*.html; separate diff forthcoming.

Reviewers: eater, alpert, john

CC: csilvers, peter

Differential Revision: http://phabricator.khanacademy.org/D2145