Suppress warning by using local ember-source

When running `bundle exec rake`, the following warning is shown:

```
/Users/tricknotes/.rbenv/versions/2.0.0-dev/lib/ruby/gems/2.0.0/gems/ember-source-0.0.8/lib/ember/version.rb:2: warning: already initialized constant Ember::VERSION
/Users/tricknotes/Documents/dev/ember.js/lib/ember/version.rb:2: warning: previous definition of VERSION was here
```

This cause is that `ember-source` is already required before [`require './lib/ember/version.rb'`](https://github.com/emberjs/ember.js/blob/master/Rakefile#L3).
To avoid this, I used locale `ember-source` as dependency gem.