Allow `application.js` injection in more cases.

Previously, when the `ember:bootstrap` generator was run it would only inject
the template if `require_tree` was found within the existing file. This meant
that if the file was empty (ala `rails-api`) or simply didn't call
`require_tree` the generator was silently failing.

Resolves https://github.com/emberjs/ember-rails/issues/183.