Add hard dependency on underscore

Without this dependency listed in the dependency field, browserifying this library on its own yields this error:

`$ browserify .
Error: module "underscore" not found from "backbone.marionette/lib/core/amd/backbone.marionette.js"`

... since `require('underscore')` is explicitly called in the UMD wrapper. This also keeps the dependencies field in sync with the fields for jam.