explicitly requiring Open3

i was getting

```
Failure/Error: Unable to find matching line from backtrace
     ActiveRecord::RecordInvalid:
       Validation failed: File Failed to thumbnail with ffmpegthumbnailer. Check ffmpegthumbnailer install and verify video is not corrupt. Original error: uninitialized constant CarrierWave::Video::Thumbnailer::FFMpegThumbnailer::Open3, File Failed to thumbnail with ffmpegthumbnailer. Check ffmpegthumbnailer install and verify video is not corrupt. 
Original error: uninitialized constant CarrierWave::Video::Thumbnailer::FFMpegThumbnailer::Open3
```

and this *may* be solved by
```ruby
require 'open3'
...
```
in `MyUploader` of my project, but it would be great if you require the needed class explicitly where it is used

thanks!