build error in ffmpeg 4.0

[citrus-lemon]

error occur in ffmpeg 4.0

src/movie.cc:70:38: error: use of undeclared identifier 'PIX_FMT_RGB24'; did you mean 'AV_PIX_FMT_RGB24'
src/movie.cc:94:5: warning: 'av_free_packet' is deprecated
src/movie.cc:65:22: error: use of undeclared identifier 'avcodec_alloc_frame'

send pull requests #52
but Travis CI refused because of using the earlier ffmpeg

[citrus-lemon]

program also occurs in #49