compilation fix: don't use deprecatedAVCODEC_MAX_AUDIO_FRAME_SIZE

Hi there,
I came across the following compile error:
```
~essentia/src/algorithms/io/audioloader.h:42:42: error: ‘AVCODEC_MAX_AUDIO_FRAME_SIZE’ was not declared in this scope
```
it seems that the AVCODEC_MAX_AUDIO_FRAME_SIZE macro is deprecated, ffmpeg(ffmpeg 2.0.1) on my platform(may be the latest version) doesn't support it any more.

ref: http://ffmpeg.org/pipermail/ffmpeg-cvslog/2012-August/053785.html