Update subsonic to 6.1.5.

[studersi]

Building the Docker container for Subsonic does not work anymore. It fails with the following error:

ERROR: Service 'subsonic' failed to build: The command '/bin/sh -c wget http://johnvansickle.com/ffmpeg/releases/ffmpeg-release-64bit-static.tar.xz -O /tmp/ffmpeg-release-64bit-static.tar.xz &&   mkdir -p /opt/ffmpeg &&   tar xf /tmp/ffmpeg-release-64bit-static.tar.xz -C /opt/ffmpeg --strip-components=1' returned a non-zero code: 8

This pull request fixes the link to the static ffmpeg resource and bumps the subsonic version to 6.1.5.

Your Subsonic Docker setup is much appreciated. I'm running the Dockerfile as proposed by this pull request myself (different docker-compose.yml though but also version 6.1.5).

I did not update the file README.md.

[studersi]

I just noticed @albal beat me to it and already bumped the version number and updated the README: #3.

I rebased against his branch, so this pull request fixes the ffmpeg link in addition to his changes.