support mp3gain alternative

[bgemmill]

Since mp3gain was dropped from debian due to being unmaintained, from Ubuntu 16.04 onwards it's only available from unofficial sources: https://launchpad.net/~flexiondotorg/+archive/ubuntu/audio

Of the alternatives, it looks like bs1770gain is the most recently maintained as of 2017: http://bs1770gain.sourceforge.net/

I would have suggested rgain as well, but that also looks abandoned as of mid 2016: https://pypi.python.org/pypi/rgain