Update to latest version of stefanpenner/ember-app-kit

This is the result of the follwing command, inspired by
http://git.io/axlwRg :

    git remote add eak https://github.com/stefanpenner/ember-app-kit
    git checkout --patch eak/master
    // manually decide what to merge in
Remove RSVP error handling, as reasoned here: http://git.io/tdHiNQ