Ability to work without sprockets (#671)

This enables omitting Sprockets.

Sprockets'  tasks (assets:*)  not available if you do not require 'sprockets/railtie' and when you try run some rake task got a message: 
```bash
ReactOnRails: Set generated_assets_dir to default: app/assets/webpack
Running via Spring preloader in process 18586
rake aborted!
Don't know how to build task 'assets:precompile' (see --tasks)
/Users/arthur/.rvm/gems/ruby-2.3.3/gems/react_on_rails-6.3.4/lib/tasks/assets.rake:35:in `<top (required)>'
...

```
My application config

```ruby
# frozen_string_literal: true
require_relative 'boot'

require 'rails'

# Pick the frameworks you want:
require 'active_model/railtie'
require 'active_job/railtie'
require 'active_record/railtie'
require 'action_controller/railtie'
require 'action_mailer/railtie'
require 'action_view/railtie'
# require 'sprockets/railtie'

Bundler.require(*Rails.groups)

module My
  class Application < Rails::Application
    # Settings in config/environments/* take precedence over those specified here.
    # Application configuration should go into files in config/initializers
    # -- all .rb files in that directory are automatically loaded.

    config.i18n.load_path += Dir[Rails.root.join('config', 'locales', '**', '*.{rb,yml}').to_s]
    config.i18n.default_locale = :ru
    config.i18n.enforce_available_locales = true

    config.time_zone = 'Moscow'

    config.middleware.use I18n::JS::Middleware
  end
end
```

CHANGELOG.md

@@ -5,6 +5,9 @@ Contributors: please follow the recommendations outlined at [keepachangelog.com]
 
 ## [Unreleased]
 *Please add entries here for your pull requests.*
+### Changed
+- Add ability use gem without sprockets. [#671](https://github.com/shakacode/react_on_rails/pull/671) by [fc-arny](https://github.com/fc-arny)
+
 ### Fixed
 - Fixed issue [#706](https://github.com/shakacode/react_on_rails/issues/706) with "flickering" components when they are unmounted too early [#709](https://github.com/shakacode/react_on_rails/pull/709) by [szyablitsky](https://github.com/szyablitsky)
 

lib/tasks/assets.rake

@@ -1,28 +1,50 @@
 require "react_on_rails/assets_precompile"
 
-namespace :react_on_rails do
-  namespace :assets do
-    desc "Creates non-digested symlinks for the assets in the public asset dir"
-    task symlink_non_digested_assets: :"assets:environment" do
-      ReactOnRails::AssetsPrecompile.new.symlink_non_digested_assets
-    end
+if defined?(Sprockets)
+  namespace :react_on_rails do
+    namespace :assets do
+      desc "Creates non-digested symlinks for the assets in the public asset dir"
+      task symlink_non_digested_assets: :"assets:environment" do
+        ReactOnRails::AssetsPrecompile.new.symlink_non_digested_assets
+      end
 
-    desc "Cleans all broken symlinks for the assets in the public asset dir"
-    task delete_broken_symlinks: :"assets:environment" do
-      ReactOnRails::AssetsPrecompile.new.delete_broken_symlinks
-    end
+      desc "Cleans all broken symlinks for the assets in the public asset dir"
+      task delete_broken_symlinks: :"assets:environment" do
+        ReactOnRails::AssetsPrecompile.new.delete_broken_symlinks
+      end
 
-    # In this task, set prerequisites for the assets:precompile task
-    desc <<-DESC
+      # In this task, set prerequisites for the assets:precompile task
+      desc <<-DESC
 Create webpack assets before calling assets:environment
 The webpack task must run before assets:environment task.
 Otherwise Sprockets cannot find the files that webpack produces.
 This is the secret sauce for how a Heroku deployment knows to create the webpack generated JavaScript files.
-    DESC
-    task compile_environment: :webpack do
-      Rake::Task["assets:environment"].invoke
+      DESC
+      task compile_environment: :webpack do
+        Rake::Task["assets:environment"].invoke
+      end
+
+      desc "Delete assets created with webpack, in the generated assetst directory (/app/assets/webpack)"
+      task clobber: :environment do
+        ReactOnRails::AssetsPrecompile.new.clobber
+      end
     end
+  end
+
+  # These tasks run as pre-requisites of assets:precompile.
+  # Note, it's not possible to refer to ReactOnRails configuration values at this point.
+  Rake::Task["assets:precompile"]
+    .clear_prerequisites
+    .enhance([:environment, "react_on_rails:assets:compile_environment"])
+    .enhance do
+      Rake::Task["react_on_rails:assets:symlink_non_digested_assets"].invoke
+      Rake::Task["react_on_rails:assets:delete_broken_symlinks"].invoke
+    end
+end
 
+# Sprockets independent tasks
+namespace :react_on_rails do
+  namespace :assets do
     desc <<-DESC
 Compile assets with webpack
 Uses command defined with ReactOnRails.configuration.npm_build_production_command
@@ -33,21 +55,5 @@ sh "cd client && `ReactOnRails.configuration.npm_build_production_command`"
         sh "cd client && #{ReactOnRails.configuration.npm_build_production_command}"
       end
     end
-
-    desc "Delete assets created with webpack, in the generated assetst directory (/app/assets/webpack)"
-    task clobber: :environment do
-      ReactOnRails::AssetsPrecompile.new.clobber
-    end
   end
 end
-
-# These tasks run as pre-requisites of assets:precompile.
-# Note, it's not possible to refer to ReactOnRails configuration values at this point.
-Rake::Task["assets:precompile"]
-  .clear_prerequisites
-  .enhance([:environment, "react_on_rails:assets:compile_environment"])
-  .enhance do
-    Rake::Task["react_on_rails:assets:symlink_non_digested_assets"].invoke
-    Rake::Task["react_on_rails:assets:delete_broken_symlinks"].invoke
-  end
-