Install image compression tools on Jenkins

Summary:
The new webapp-maintenance job will run image compression tools on our
images weekly, to make sure they remain optimized. But, the tools need
to be installed first. That's what this change is about.

Test Plan:
I ran the install_image_tools function on Jenkins and it worked, then I
ran it again and it didn't even try to install pngout.

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D11195