Simplify Carrierwave calls to remove `manipulate!` wrapper

A few times now, I've run across some strange behavior with regards to how Carrierwave images were being reprocessed - specifically, while optipng and/or pngquant were getting called, the optimized images weren't getting saved properly and reuploaded when using S3. The oddness seemed to be related to the call to `manipulate!` and the file moving that happens around that. I dug into some of the Carrierwave documentation and a few other examples of custom processors, and it seems like for the use case in Piet `manipulate!` is not required - we're not doing any image manipulation via rmagick/minimagick.

Anyway, I've been running a patched version of Piet with these changes in production for a while now, and everything seems to be working great. Figured I'd commit them upstream as well.