Fix broken generator for ember-data

The channel fallback for ember-data doesn't work correctly.
`release -> beta` is expected but actual `release -> release`.

This cause is that the following PRs have incompatible changes:
* https://github.com/emberjs/ember-rails/pull/403
* https://github.com/emberjs/ember-rails/pull/404
Merge pull request #407 from tricknotes/fix-broken-generator

Fix broken generator for ember-data