Unpin ember-data version

[rwjblue]

Once emberjs/data#5900 lands in a released version of ember-data (likely 3.10.0-beta.1) we should update to use the released / tagged version of ember-data.

[NullVoxPopuli]

ember data pinned here: https://github.com/ember-cli/ember-octane-blueprint/blob/master/packages/%40ember/octane-app-blueprint/files/package.json#L37