Update to Ember 1.1.0 (and fix tests).

Updated Gemfile to use 1.1.0 (latest stable version of Ember).

Ember 1.1.0 does not allow items to be re-registered on a container if they have already been looked up. This change was made in [b8b9878bc](https://github.com/emberjs/ember.js/commit/b8b9878bcf6517ee2b25b0e309f2a5bf5e3ff7d1).

This refactors the tests for `DS.Store#push` so that it does not throw any errors with 1.1.0.