Augment the blacklist to hold _hashed-* files.

Summary: Add a legitimate lint error found by the lint check.

Test Plan:
Fingers crossed.

Ran ~/khan/devtools/khan-linter/runlint.py in khan-exercises.

Reviewers: alpert, eater

Reviewed By: alpert

CC: chris

Differential Revision: http://phabricator.khanacademy.org/D2652