index.js: use app argument instead of this.app

[jonathanong]

fixes #103

[davewasmer]

@jonathanong would you mind linking to the docs that describe app.app here? I'd love to understand a bit more about why this change is necessary.

[jeffjewiss]

I don't know if this is well documented, but I'm pretty sure the issue here is related to nesting add-ons. Since add-ons can be nested n levels deep, the reference to app could mean the user's application, or it could mean a parent add-on.

The fix in this PR has been used in several add-ons to ensure that the correct configuration is being referenced. Here is another example: rwjblue/ember-cli-pretender@dda9bf8

It should be safe to merge this once the tests are fixed. 😄