if assets.enabled is left blank, the default is now to use the asset pipeline

The default behavior in Rails now is to use the asset pipeline.  The existing conditional in the railtie is `!!(::Rails.configuration.assets.enabled)`.  This actually evaluates to false for nil -- which essentially overrides the default behavior to use the legacy helpers. 

nil / left blank for assets.enabled should use the asset pipeline.  The engine railtie conditional is only run for Rails 3.2 + so we should be able to safely assume asset pipeline is the default.