[dm-constraints] don't migrate constraints for remixable modules
[dm-constraints] don't migrate constraints for remixable modules

* Since remixable modules need to include DataMapper::Resource,
  dm-constraints tried to migrate constraints for the module
  and not the generated model. However, there is no storage
  for the module and so the constraint migration used to fail.

* Also, I noticed that currently 36 out of the 94 specs for
  dm-constraints fail all with the same message (at least on
  my machine)

    constant Object::Article not defined
    ../spec_helper.rb:42:in `remove_const'

  This happens with or without this patch.

[#961 state:resolved]

datamapper/dm-constraints