Change lower limit of video bitrate from 100kbps to 64kbps

If we use high quality audio streaming by mp3 320kbps at NicoNicoLive
streaming service in Japan,
since the total bit rate has been limited to 384kbps, We hope to change
lower limit of video bitrate from 100kbps to 64kbps.
Merge pull request #344 from shousa/master

Change lower limit of video bitrate from 100kbps to 64kbps