[Patch] rake assets:compile errors when asset pipelining disabled

Currently in master, running rake assets:compile with config.assets.enabled = false results in

undefined method `precompile' for nil:NilClass

This patch raises an error if asset pipelining is disabled.

I chose to raise, as i think if this task was part of a deploy process (and i think that would be the general use case), and assets were accidentally turned off, you would want it to abort the deploy.

I would love feedback if you think this is the wrong approach :)