From the report:
  This patch fixes a bug that rig modules of 'arm' and
  'leg' cannot be loaded failing as follows:
  
  Rigify: No module named 'rigify.rigs.biped.arm.biped'
  Rigify: No module named 'rigify.rigs.biped.leg.biped'

I couldn't reproduce the error, but new code works on my system
just as well as the old code, so I assume this serves to make the
code more robust on a larger range of systems.

Thanks Shinsuke!