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.
Merge pull request #294 from rjackson/handle_more_cases_for_application_injection

Allow `application.js` injection in more cases.