Sprockets 4 compatibility

[kennyadsl]

Sprockets 4 requires a configuration manifest file to be created in the application. This is fine for new stores but it's not working with our DummyApp at the moment since it is an in-memory application, and not a real application on the filesystem, like most engines do in spec/dummy with rake test_app.

At the moment, Sprockets 4 is not able to allow setting a custom location for this configuration manifest file and it is looking for our manifest in spec/dummy/app/assets/config/manifest.js, which does not exist. Spec fails consequently.

I'm trying to propose a change to sprockets-rails that allows setting a custom location for the manifest with this PR, even because there's a Rails config named config.assets.manifest, which I think is only used for this reason, but I'm not super sure since I can't find a lot of documentation around.

The only alternative solution I can think about is locking Sprockets to 3.x on all the currently supported Solidus versions.

[elia]

Fixed by #3379