This updates the Rollup configuration for Spina to take in Backbone

and underscore as externals. This will pass in Backbone and _ in

the UMD wrapper for the module, taking them the root object (such as

window) or from the calling module.
Defining these as externals is important for module resolution, and to

avoid the assumption of what provides these modules.

Verified that the resulting UMD wrapper takes in Backbone and _ as

parameters and that they're populated automatically.
Also verified that I could successfully import these into a module

bundling this with Backbone, and that resolution worked correctly.