Fixes assets:precompile for new Rails 4 apps: fix Sprockets check

If you create a new Rails 4 app (rather than upgrade), `rake assets:precompile` will fail because `config.assets.enabled` was deprecated and is no longer set by default - thus evaluates to `nil`, triggering the guard clause in the diff.

The recommended way to disable asset pipeline is to remove the `sprockets-rails` gem, so I figured we can just check if `Sprockets` is defined.

Reference: http://yetimedia.tumblr.com/post/33320732456/moving-forward-with-the-rails-asset-pipeline