Fix paperclip specs

This should fix the specs that were broken by the Paperclip upgrade. I made one first attempt that worked, but it felt weird to be modifying mime types. What I realized is that there's no reason to be uploading `.rb` files. I believe those were leftover from days past when I believe Surveyor was actually an engine running within the same application. So I changed them to a `.txt` file, and the specs pass now.

The `to_file` method was deprecated and removed, so I had to change that syntax as well.