[#91428] Upgrade Rails and Paperclip

After running bundler-audit, this upgrades Rails to 3.2.21 and
Paperclip to 4.2. There were some changes described in
https://github.com/thoughtbot/paperclip/blob/master/UPGRADING but we
were already setting :path and :url. When testing, we need to make sure
to verify that old uploads are still available for download.

Closes #227