Comment out the 'You are missing PIL' warning.

Summary:
I think they do the right thing by silently setting the images-stub to
be a 'not-implemented' stub, so you get an error when you try to
actually use the GAE images library.

(That error even says "install PIL" so there's really *nothing* to be
gained by having this on-init warning.  Ah well.  Good thing we've
forked the source. :-) )

This will avoid log spam, and also avoid confusion, especially from
new folks, who think this is actually an error that might be causing
them problems.

Test Plan:
Ran
   make serve
and didn't see any mention of PIL.

Reviewers: dylan

Reviewed By: dylan

Subscribers: ayman, dylan

Differential Revision: http://phabricator.khanacademy.org/D13099