Update jamjs exports config

As `LayoutManager` is deprecated in favour of `Backbone.Layout`, the jamjs config should be updated too.

This would fix a part bug we had in Backbone-Boilerplate: https://github.com/tbranyen/backbone-boilerplate/issues/199

(I guess to fix this someone need to also republish the fixed version to Jam package repo)
Merge pull request #264 from SBoudrias/patch-1

Update jamjs exports config