Do not pass watermark option to ffmpeg because it is processed wrong

After https://github.com/streamio/streamio-ffmpeg has added own way to add watermark option `watermark` is processed with wrong way. So we should remove this option from options list before pass them to ffmpeg.

It closes https://github.com/rheaton/carrierwave-video/issues/23

It will be better in future to use native way to apply watermark.