Ensure precompile setting from app is applied to rake task.

OK - taking another swing at this with a different approach.

I was not able to use the precompile setting to solve the situation described here:

https://github.com/rails/sprockets-rails/pull/34

This is due to the details below:

* Rails calls `load_tasks` prior to the app's configure block.
* This results in the sprockets rake task being initialized without the app's assets.precompile setting.
* This change stores a reference to the sprockets rake task so the assets can be updated with the properly configured assets.precompile setting.

However, with this change, loose assets in lib or vendor from a legacy gem will now be picked up if they are explicitly specified in the precompile setting.