Optimize audio file for NyanCatMusicFormatter

Replace 8.7 MB MP3 file with a 433 KB version. The smaller file is also shorter (27 seconds vs. 217 seconds), so I'm playing it in a loop.

This reduces the total gem size from 8.3 MB to 1.0 MB.

I've also rewritten the `if osx?` and `if linux?` statements as a single `if`...`elsif` statement, since they should be mutually exclusive.