fixup for s3_server_side_encryption option

Per [S3 documentation](http://docs.aws.amazon.com/AmazonS3/latest/dev/SSEUsingRubySDK.html), the `server_side_encryption` option value should be `:aes256`, not `AES256`.

This PR fixes this option, so that
```
has_attached_file :file, s3_permissions: :private, s3_server_side_encryption: :aes256
```
results in server side encryption being used for storage of the S3 objects, which currently is not the case with the above attachment configuration. I confirmed this by using this commit and the above configuration, and verifying that the S3 console shows the file is stored with SSE.