Set prototype extensions to false by default for addons

[bcardarella]

See: https://github.com/dockyard/ember-validations/blob/master/tests/dummy/config/environment.js#L10

In ember-validations I have disabled prototype extension for the test suite. I believe that all addons should be developed with prototype extensions turned off to ensure the widest compatibility with how Ember is being used.

I don't have the time to add this into ember-cli, if anyone else wants to tackle that would be great. I'm not sure how best to accomplish this but the addon generator should be relying upon the config/environment.js from the app generator then modify the EmberENV object in the file to disable prototype extensions rather than generating its own file.

[hhff]

👍

[stefanpenner]

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