r-cran-dosnow fix for 700498 cannot migrate to testing

[Niels Thykier <niels@thykier.net>]

Hi Dirk,

The fix for #700498 is currently stuck in unstable due to a dependency
on a versioned dependency on r-base (or an binary built by r-base).  At
this point we are not ready to accept the changes to r-base for Wheezy.
  So, I was hoping that we convince you to upload a fix for #700498 to
testing-proposed-updates.

~Niels