[PULL REQUEST] Respect 'config.digest = false' when manifest.yml exists

Hello everyone,

Can someone have a quick look at this pull request and tell me if I am maybe barking up the wrong tree?

Basically, it would seem that if one precompiles an application's assets, thus creating a manifest.yml file, the value of 'config.digest' is ignored.

More details: https://github.com/rails/rails/pull/3135