Rename application.rb template application.rb.tt

This avoids issues with tooling (like YARD) trying to parse this as
ruby, and getting confused when it encounters an erb tag.

.rb.tt is the extension Rails uses for these types of files.

core/lib/generators/spree/dummy/dummy_generator.rb

@@ -53,7 +53,7 @@ def test_dummy_config
 
       template "rails/database.yml", "#{dummy_path}/config/database.yml", force: true
       template "rails/boot.rb", "#{dummy_path}/config/boot.rb", force: true
-      template "rails/application.rb", "#{dummy_path}/config/application.rb", force: true
+      template "rails/application.rb.tt", "#{dummy_path}/config/application.rb", force: true
       template "rails/routes.rb", "#{dummy_path}/config/routes.rb", force: true
       template "rails/test.rb", "#{dummy_path}/config/environments/test.rb", force: true
       template "rails/script/rails", "#{dummy_path}/spec/dummy/script/rails", force: true