Compatibility with Rails 4.0.0's asset pipeline config

`config.assets.enabled` has been deprecated, so when you create a new Rails 4 app, it returns `nil`. Because of this, roadie thinks that assets are not enabled on a Rails 4 app. 

However, when you create a Rails 4 app without assets (e.g. `rails new Blog --skip-sprockets`), `config.assets.enabled` does get explicitly set to `false`. 

So, I updated the asset pipeline check to explicitly make sure it's not false, rather than truthy. In the future, Rails may leave `config.assets.enabled` as `nil` and we'll have to figure out a workaround then (e.g. seeing if `Sprockets` is loaded, etc.).

Fixes #58.