Add the rails_helper when creating a new app

With the RSpec 3.0.0 we have some new configs and the rspec-rails
generates now a `rails_helper.rb`.

With this we create both files with the HE:labs config.

- move rails configs to rails_helper.rb
- remove the `config.fixture_path = "#{::Rails.root}/spec/fixtures"` since
we use factories
- remove the `config.infer_base_class_for_anonymous_controllers = false`
since is a good thing to have
- Dont add `config.expose_dsl_globally = false` since we have the same
behavior with `config.disable_monkey_patching!`