Updated MP3 codec mime-type to be RFC 3003 compatible

In recent versions of Chrome adding the `codecs=“mp3”` part to the
mime-type causes the `canPlayType` function to return a falsy value.

To fix this I’ve removed the codec part from the mime-type, which
should fix any issues with feature detection.

Related bug report:
https://code.google.com/p/chromium/issues/detail?id=386073