Generating test_apps for extensions doesn't work the way the developer extensions tutorial implies it should

This pull request fixes problems with the generation of extensions.

Before this changeset, it was not possible to generate a working dummy test_app for extensions due to the following:

* The config/application.rb file in the extensions dummy test_app did not require spree, therefore the db:migrate step failed.
* The common:test_app rake task used a faulty algorithm to discover the extension's InstallGenerator.
* When running the InstallGenerator the test_app task would prompt the user whether to run the migrations.

The changes in this PR fix these problems, while minimising changes to the DummyGenerator so that working test_apps may still be generated for spree components (e.g. spree_frontend etc)