Update requirement from PIL to Pillow

[rehandalal]

tiny r?

[willkg]

My concern here is that we use this file to set up the servers. So if we change this, how does that affect setting up new servers on RHEL?

[rehandalal]

@willkg I think it should be fine based off of:
http://pillow.readthedocs.org/en/latest/installation.html#platform-support

[rehandalal]

@willkg thoughts?

[willkg]

So there are two paths here:

1. we create a webops bug to install pillow on our dev server, make sure it's possible and works and then update our requirements confident that we're not creating a situation where our server reality is different than our development reality
2. we make this change and figure out the server side of the change at some later point knowing that we're introducing yet another difference between server and development environments

I talked with one of the pillow devs at pycon 2014. Previously I thought pillow was still taking a very conservative "only changes to compilation stuff" approach. They're not anymore (and might not have been for some time), so there will definitely be differences. I have no idea if we'll hit them.

[dean]

Since we're switching all of our requirements out of /vendor to peep install I think we should make the change for PIL to Pillow at the same time. We're going to need heavy testing to ensure everything still works when making the switch to peep so I think it'd be a good idea to include this when we do that testing.

[rehandalal]

r+'d by @rlr on IRC

[rehandalal]

travis is failing because it cannot install PIL, so landing this to prevent travis failures

[rehandalal]

e1712e0