use load-initializer vendor'd package

As per discussion on  https://github.com/stefanpenner/ember-app-kit/pull/588 use https://github.com/stefanpenner/ember-load-initializers also in `ember-cli`

I'm kinda new to ember-cli so let me know if this is ok or if there is some path to test this kind of changes to the blueprint (that is not try the generated app)

thanks

/cc @stefanpenner