Behave like AR does wrt migrating dev and test storages
Behave like AR does wrt migrating dev and test storages

This change makes some rake tasks behave like so:

  rake db:create      # both dev and test repos
  rake db:drop        # only the current env repo
  rake db:automigrate # only the current env repo
  rake db:autoupgrade # only the current env repo
  rake db:migrate     # only the current env repo

Previously, db:drop/automigrate/autoupgrade were
operating on both dev and test repos if RAILS_ENV
was set to development.

While creating both dev and test repos when no
RAILS_ENV is specified (defaults to development)
makes sense, it doesn't make sense to perform the
other operations on both repo envs too.

[#1326 state:resolved]
datamapper/dm-rails