Fixes issue #353

After using THUMBNAIL_DEBUG, I found the error:

The problem is, that ``image.__crop`` contained floats for the box[2] and box[3] fields. This during conversion fired an InvalidType exception by Pillow. I then saw that if ``crop=True``, the box[2] and box[3] fields were ints. Therefore I fixed that part in your code.

If you accept this patch, it would be great if you could release a new minor version soon, otherwise I have to install all my projects through GitHub instead of PyPI.