Using codec->time_base is deprecated to specify muxer settings.
Resolves issue with FPS value for AVI files with FFmpeg 4.0.

Related FFmpeg commits:
- https://github.com/FFmpeg/FFmpeg/commit/194be1f43ea391eb986732707435176e579265aa
- https://github.com/FFmpeg/FFmpeg/commit/91736025b2807995e29bd0661807c1c84f515fef