Setup NPM with an empty package.json file

Note that, even if we're excluding the `node_modules/` folder from
version control, we aren't adding it to Capistrano's shared folders
because, when `node_modules` is a symbolic link, NPM removes it when
running `npm install`.

.gitlab-ci.yml

@@ -23,6 +23,7 @@ tests:
     - bundle --without development
     - bundle exec rake db:setup
     - bundle exec rake assets:precompile > /dev/null 2>&1
+    - npm clean-install
     - bin/knapsack_pro_rspec
   artifacts:
     when: on_failure