Auto merge of #6495 - joeyespo:functional-start-app, r=nathanhammond
Auto merge of #6495 - joeyespo:functional-start-app, r=nathanhammond

Make startApp functional

Instead of using `let`, then setting the closure of `application` inside an immediately-run function callback, return the new application object, and then return that result directly.

This looks like a relic of [upgrading from `var`](https://github.com/ember-cli/ember-cli/commit/cc65668f9065d300e0daee49a3a201bfcca9c663) and the dated "put all `var` declarations at the top" [convention](https://github.com/ember-cli/ember-cli/commit/18a5f621b3f41b3a5f4139fc3d38118cde5e11df#diff-06761a3424465f8bb1a3e0fe86b96c31).

Note that if `Ember.run` ever becomes an asynchronous function, startApp will return `undefined` even without this change. `Ember.run` [does](https://github.com/emberjs/ember.js/blob/v2.10.0/packages/ember-metal/lib/run_loop.js#L76-L78) run [immediately](https://github.com/ebryn/backburner.js/blob/v0.3.1/lib/backburner/index.js#L151) and [returns what is returned](https://github.com/ebryn/backburner.js/blob/v0.3.1/lib/backburner/index.js#L196) from the callback.

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