Now supporting Paperclip 4.1

Howdy,

Using this on a new project and wanted to use the latest version of Paperclip (4.1). There were some issues that I believe I've resolved. In particular, the requirement that all models with attachments validate the content type.

In truth, most of the issues I had to fix were related to tests. The Paperclip API has changed some and adjustments needed to be made.

I've also made the ImageMagick test comparisons a little more forgiving. All of the comparison tests were failing for me due to a 0.004 percent difference. I chalked this up to different versions of ImageMagick doing cropping a little differently and am now rounding to two decimal places. It will still fail if the crops are done wrong, but very subtile differences between ImageMagick versions should no longer cause tests to fail.

Let me know if you need anything else done. Looking forward to getting this PR merged so I can stop using my own fork.

Keep well,
Emerson