Pin Sprockets version in CI

[delner]

Tests in CI started breaking with Sprockets expecting a manifest JS file:

18: from /app/test/contrib/rails/apps/rails5.rb:8:in `<top (required)>'
...
1: from /usr/local/bundle/gems/railties-5.2.3/lib/rails/initializable.rb:32:in `instance_exec'
/usr/local/bundle/gems/sprockets-rails-3.2.1/lib/sprockets/railtie.rb:105:in `block in <class:Railtie>': Expected to find a manifest file in `app/assets/config/manifest.js` (Sprockets::Railtie::ManifestNeededError)
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

Since we don't use or test sprockets, let's disable it in CI to prevent build failures.

[delner]

Opened a related issue here: rails/sprockets-rails#444

[delner]

Looks good @marcotc; thanks for updating this!