Fix for #938

As per #938 and many other reported bugs on 3rd party libraries i.e. https://github.com/Knockout-Contrib/Knockout-Validation/issues/259

Because some of the third party libraries such as knockout validation are not aware of `AMD` we need to always export `ko` to the global namespace.
Otherwise when optimizing multiple modules with AMD including knockout itself, the other 3rd party modules would simply fail because they would always rely on the global `ko` variable.
This pull request easily fixes that case, and is also making sure the define is not anonymous, so that the shim configuration can be used side-by-side with the default ko module.