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`.

bin/setup

@@ -17,6 +17,9 @@ FileUtils.chdir APP_ROOT do
   system! "gem install bundler --conservative"
   system("bundle check") || system!("bundle install")
 
+  # Install JavaScript dependencies
+  system!("npm install")
+
   puts "\n== Copying sample files =="
   unless File.exist?("config/database.yml")
     FileUtils.cp "config/database.yml.example", "config/database.yml"