pointing to spree 2 gem

this is for **2-0-stable** branch
- updated spree reference to latest v. **2.0.4** release in gemspec, as suggested in TODO
- added **spree_auth_devise** in *Gemfile* as generated spree extensions do, in order to have *spree_frontend* and the other engines available.
- updated `common:test_app` task to `extension:test_app`, which does include those engines in dummy app's **config/application.rb**

with this, dummy app is correctly generated and all tests pass