Make the meme cache case-sensitive

Summary:
Fixes T13172. At one point we always capitalized all the text, and the cache uses capitalized text.

However, we stopped capitalizing the text at some point. Modern memes are more more subtle than old memes, and when we eventualy add support for things like "explodey brain" we'll certainly want to support mixed case.

Practically, this stops you from changing the capitalization of a cached meme. Get rid of the cache transform.

Test Plan:
none lul

(I don't have gd installed locally and buiding it requires building libjpeg and libpng or giving up and using brew. I'l vet this in production.)

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13172

Differential Revision: https://secure.phabricator.com/D19537