Don't use the setuid/setgid stanzas in the exercise-screens upstart job.

Summary:
Those stanzas were introduced in Upstart 1.4 (ships with Ubuntu 12.04) but
ka-ci is running Upstart 1.3 (Ubuntu 11.10).

Amend the migration plan as follows:
- Remove old symlink in /etc/init.d
- Add new symlink (spell the file properly now!)
- Start the service.

Test Plan: Tested it on ka-ci itself.

Reviewers: dylan, alpert

Reviewed By: dylan

CC: stephanie

Differential Revision: http://phabricator.khanacademy.org/D2057