Make sure that khan-linter requirements are installed.

Oops, forgot that I had called "arc diff" on my own fork.

Summary: Fixed path to khan-linter directory.

Test Plan: Ran the single added command; "arc diff" no longer fails.

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D2246