`ulimit` does not understand memory multipliers

[mschubert]

From discussion in https://github.com/ropensci/drake/issues/813:

The SLURM templates understand memory limits in Mb or Gb, e.g. by supplying "5G" or "128M".

ulimit does not understand that, however. Probably should parse those simple examples once I handle the memory limit from within clustermq.