8.0.0

Improvements:

• Added support for retries in the FFMPEG::Transcoder class. This allows for more robust command
  argument composing and thus more stable outputs.

Breaking Changes:

• The FFMPEG::Transcoder#process! method will now fail if the expected output files do not exist after
  successful processing. This behaviour can be controled by passing checks: [] to the transcoder
  initializer.
• The FFMPEG::Status::ExitError class has been renamed to FFMPEG::ExitError.
• The FFMPEG::ExitError class now holds a reference to the StringIO output of the FFmpeg command
  (before it contained the String representation).