Use Sprockets 4 for app usage

This updates the Gemfile.lock so that this app uses the current version
of Sprockets when it's running directly as an app.

I needed to add an app/assets/config/manifest.js to get around an
error when I'd run `rake app:assets:precompile`:

```
➜  govuk_publishing_components git:(backwards-compatible-sprockets-4) ✗ rake app:assets:precompile
rake aborted!
Sprockets::Railtie::ManifestNeededError: Expected to find a manifest file in `app/assets/config/manifest.js`
But did not, please create this file and use it to link any assets that need
to be rendered by your app:

Example:
  //= link_tree ../images
  //= link_directory ../javascripts .js
  //= link_directory ../stylesheets .css
and restart your server

For more information see: https://github.com/rails/sprockets/blob/070fc01947c111d35bb4c836e9bb71962a8e0595/UPGRADING.md#manifestjs
/Users/kevin.dew/govuk/govuk_publishing_components/spec/dummy/config/environment.rb:5:in `<top (required)>'
Tasks: TOP => environment => app:environment
(See full trace by running task with --trace)
```

This error is peculiar as the app having it's assets precompiled is in
spec/dummy and has it's own manifest.js in spec/dummy/app/assets/config.
Perhaps it's a bug?

Gemfile.lock

@@ -337,7 +337,7 @@ GEM
     sentry-ruby-core (4.5.2)
       concurrent-ruby
       faraday
-    sprockets (3.7.2)
+    sprockets (4.0.3)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
     sprockets-rails (3.4.2)

app/assets/config/manifest.js

@@ -0,0 +1,5 @@
+// This file is needed for govuk_publishing_components to support
+// app:assets:precompile with Sprockets 4, it will error without it.
+//
+// See govuk_publishing_components_manifest.js for the list of assets to be
+// pre-compiled.