Fix to update generated relation name for model generator

From Ember Data 1.0.0.beta,
the relation should be defined as underscored name.

Before:
``` js
App.Comment = DS.Model.extend({
  post: DS.belongsTo('App.Post')
});
```

After:
``` js
App.Comment = DS.Model.extend({
  post: DS.belongsTo('post')
});
```