Make the security updates really happen.

Summary:
I was checking the logs and surprised to find that we hadn't been
doing any security updates since the very first time.

Investigating deeper, I discovered I need to run 'apt-get update'
first.  So now I do.

Fix the comment to reflect what we're actually doing.

Also, it turns out we *do* care if 'apt-get update' fails; it
shouldn't normally do that.  So I took out the '|| true'.  We'll see
how it goes...

Test Plan:
Fingers crossed.
But I ran 'apt-get update' followed by /etc/cron.daily/security-update
on toby, and it updated a bunch of stuff.

(same)

Reviewers: chris

Reviewed By: chris

Differential Revision: http://phabricator.khanacademy.org/D14541