up and down migrations allow version to be specified by task argument
up and down migrations allow version to be specified by task argument

This results in a cleaner looking commandline:
  rake db:dm:migrate:up[2]
cf.
  rake db:dm:migrate:up VERSION=2
And also draws attention to the fact a version can be specified, at it is
included in the output of rake -T

mattetti/dm-more,mattetti/dm-more,cachafla/dm-more,bterlson/dm-more,bterlson/dm-more,merbjedi/dm-more-hacked,merbjedi/dm-more-hacked