Fix stopping slider when transition is complete

[balinterdi]

The router service exists since Ember 2.15 so my fix should work for all versions this add-on supports except for 2.12 LTS.

I left this._router as a fallback. I'm not sure if it exists in some Ember versions (it was undefined in an Ember 2.18 app) but I didn't want to break something which might work.